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/diagnostics/tests/multiplatform/expectActualClassesAreExperimentalWarning.fir.kt

@@ -0,0 +1,49 @@
+// ENABLE_EXPECT_ACTUAL_CLASSES_WARNING
+// MODULE: m1-common
+// FILE: common.kt
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>expect<!> class Clazz {
+    class <!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>Nested<!>
+
+    fun memberFun()
+    val memberProp: Clazz
+}
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>expect<!> interface Interface
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>expect<!> object Object
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>expect<!> annotation class Annotation
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>expect<!> enum class Enum
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>expect<!> class ActualTypealias
+
+expect fun function()
+
+expect val property: Clazz
+
+// MODULE: m2-jvm()()(m1-common)
+// FILE: jvm.kt
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> class Clazz {
+    <!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> class Nested
+
+    actual fun memberFun() {}
+    actual val memberProp: Clazz = null!!
+}
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> interface Interface
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> object Object
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> annotation class Annotation
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> enum class Enum
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> typealias ActualTypealias = ActualTypealiasImpl
+
+class ActualTypealiasImpl
+
+actual fun function() {}
+
+actual val property: Clazz = null!!

compiler/testData/diagnostics/tests/multiplatform/expectActualClassesAreExperimentalWarning.kt

@@ -0,0 +1,49 @@
+// ENABLE_EXPECT_ACTUAL_CLASSES_WARNING
+// MODULE: m1-common
+// FILE: common.kt
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING, EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING{JVM}!>expect<!> class Clazz {
+    class <!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING, EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING{JVM}!>Nested<!>
+
+    fun memberFun()
+    val memberProp: Clazz
+}
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING, EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING{JVM}!>expect<!> interface Interface
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING, EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING{JVM}!>expect<!> object Object
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING, EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING{JVM}!>expect<!> annotation class Annotation
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING, EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING{JVM}!>expect<!> enum class Enum
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING, EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING{JVM}!>expect<!> class ActualTypealias
+
+expect fun function()
+
+expect val property: Clazz
+
+// MODULE: m2-jvm()()(m1-common)
+// FILE: jvm.kt
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> class Clazz {
+    <!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> class Nested
+
+    actual fun memberFun() {}
+    actual val memberProp: Clazz = null!!
+}
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> interface Interface
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> object Object
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> annotation class Annotation
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> enum class Enum
+
+<!EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING!>actual<!> typealias ActualTypealias = ActualTypealiasImpl
+
+class ActualTypealiasImpl
+
+actual fun function() {}
+
+actual val property: Clazz = null!!