Mark expect/actual classifiers as experimental

^KT-61573 Fixed
Review: https://jetbrains.team/p/kt/reviews/11969/timeline

Tests:
- MultiPlatformIntegrationTestGenerated
- CliTestGenerated
- MultiPlatformIntegrationTestGenerated
- DiagnosticTestGenerated.Multiplatform
- FirLightTreeOldFrontendDiagnosticsTestGenerated

Also add -Xexpect-actual-classes flag to all necessary ./libraries/* modules
Otherwise compilation of those modules failes because of `-Werror`

compiler/testData/cli/js/jsExtraHelp.out

@@ -87,6 +87,8 @@ where advanced options include:
                              The corresponding calls' declarations may not be marked with @BuilderInference.
   -Xklib-enable-signature-clash-checks
                              Enable the checks on uniqueness of signatures
+  -Xexpect-actual-classes    The expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature.
+                             Kotlin reports a warning every time you use them. You can use this flag to mute the warning.
   -Xexpect-actual-linker     Enable experimental expect/actual linker
   -Xexplicit-api={strict|warning|disable}
                              Force compiler to report errors on all public API declarations without explicit visibility or return type.

compiler/testData/cli/jvm/extraHelp.out

@@ -178,6 +178,8 @@ where advanced options include:
                              The corresponding calls' declarations may not be marked with @BuilderInference.
   -Xklib-enable-signature-clash-checks
                              Enable the checks on uniqueness of signatures
+  -Xexpect-actual-classes    The expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature.
+                             Kotlin reports a warning every time you use them. You can use this flag to mute the warning.
   -Xexpect-actual-linker     Enable experimental expect/actual linker
   -Xexplicit-api={strict|warning|disable}
                              Force compiler to report errors on all public API declarations without explicit visibility or return type.

compiler/testData/cli/jvm/firMultiplatformCompilationWithPsiWithoutErrors.out

@@ -9,4 +9,10 @@ as no stability/compatibility guarantees are given on
 compiler or generated code. Use it at your own risk!
 
 warning: language version 2.0 is experimental, there are no backwards compatibility guarantees for new language and library features
+compiler/testData/cli/jvm/firMultiplatformCompilationWithoutErrors/common.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+expect interface A {
+^
+compiler/testData/cli/jvm/firMultiplatformCompilationWithoutErrors/jvm.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+actual interface A {
+^
 OK

compiler/testData/cli/jvm/firMultiplatformCompilationWithoutErrors.out

@@ -9,4 +9,10 @@ as no stability/compatibility guarantees are given on
 compiler or generated code. Use it at your own risk!
 
 warning: language version 2.0 is experimental, there are no backwards compatibility guarantees for new language and library features
+compiler/testData/cli/jvm/firMultiplatformCompilationWithoutErrors/common.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+expect interface A {
+^
+compiler/testData/cli/jvm/firMultiplatformCompilationWithoutErrors/jvm.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+actual interface A {
+^
 OK

compiler/testData/cli/jvm/hmpp/lowLanguageVersion.out

@@ -8,4 +8,16 @@ as no stability/compatibility guarantees are given on
 compiler or generated code. Use it at your own risk!
 
 warning: -Xfragments flag is not supported for language version < 2.0
+compiler/testData/cli/jvm/hmpp/src/a.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+expect class A {
+^
+compiler/testData/cli/jvm/hmpp/src/a.kt:5:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+expect class B {
+^
+compiler/testData/cli/jvm/hmpp/src/b.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+actual class A {
+^
+compiler/testData/cli/jvm/hmpp/src/c.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+actual class B {
+^
 OK

compiler/testData/cli/jvm/hmpp/successfulCompilation.out

@@ -8,4 +8,16 @@ as no stability/compatibility guarantees are given on
 compiler or generated code. Use it at your own risk!
 
 warning: language version 2.0 is experimental, there are no backwards compatibility guarantees for new language and library features
+compiler/testData/cli/jvm/hmpp/src/a.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+expect class A {
+^
+compiler/testData/cli/jvm/hmpp/src/a.kt:5:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+expect class B {
+^
+compiler/testData/cli/jvm/hmpp/src/b.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+actual class A {
+^
+compiler/testData/cli/jvm/hmpp/src/c.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+actual class B {
+^
 OK

compiler/testData/cli/jvm/hmpp/successfulCompilation2.out

@@ -8,4 +8,16 @@ as no stability/compatibility guarantees are given on
 compiler or generated code. Use it at your own risk!
 
 warning: language version 2.0 is experimental, there are no backwards compatibility guarantees for new language and library features
+compiler/testData/cli/jvm/hmpp/src/a.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+expect class A {
+^
+compiler/testData/cli/jvm/hmpp/src/a.kt:5:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+expect class B {
+^
+compiler/testData/cli/jvm/hmpp/src/b.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+actual class A {
+^
+compiler/testData/cli/jvm/hmpp/src/c.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+actual class B {
+^
 OK

compiler/testData/cli/jvm/k2SimpleMultiplatformGenericClass.out

@@ -1,2 +1,8 @@
 warning: language version 2.0 is experimental, there are no backwards compatibility guarantees for new language and library features
+compiler/testData/cli/jvm/k2SimpleMultiplatformGenericClass/common.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+expect class AtomicRef<T> {
+^
+compiler/testData/cli/jvm/k2SimpleMultiplatformGenericClass/jvm.kt:1:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+actual class AtomicRef<T> constructor(value: T) {
+^
 OK

compiler/testData/cli/metadata/optionalExpectationUsage.out

@@ -1 +1,4 @@
+compiler/testData/cli/metadata/optionalExpectationUsage.kt:3:1: warning: the expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. You can use -Xexpect-actual-classes flag to suppress this warning.
+expect annotation class Ann()
+^
 OK