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/test-infrastructure/tests/org/jetbrains/kotlin/test/builders/LanguageVersionSettingsBuilder.kt

@@ -103,6 +103,7 @@ class LanguageVersionSettingsBuilder {
             analysisFlag(AnalysisFlags.allowResultReturnType, trueOrNull(LanguageSettingsDirectives.ALLOW_RESULT_RETURN_TYPE in directives)),
             analysisFlag(AnalysisFlags.explicitApiMode, directives.singleOrZeroValue(LanguageSettingsDirectives.EXPLICIT_API_MODE)),
             analysisFlag(AnalysisFlags.allowKotlinPackage, trueOrNull(LanguageSettingsDirectives.ALLOW_KOTLIN_PACKAGE in directives)),
+            analysisFlag(AnalysisFlags.muteExpectActualClassesWarning, trueOrNull(LanguageSettingsDirectives.ENABLE_EXPECT_ACTUAL_CLASSES_WARNING in directives) != true),
             analysisFlag(AnalysisFlags.dontWarnOnErrorSuppression, trueOrNull(LanguageSettingsDirectives.DONT_WARN_ON_ERROR_SUPPRESSION in directives)),
 
             analysisFlag(JvmAnalysisFlags.jvmDefaultMode, directives.singleOrZeroValue(LanguageSettingsDirectives.JVM_DEFAULT_MODE)),

compiler/test-infrastructure/tests/org/jetbrains/kotlin/test/directives/LanguageSettingsDirectives.kt

@@ -64,6 +64,11 @@ object LanguageSettingsDirectives : SimpleDirectivesContainer() {
         description = "Allow compiling code in package 'kotlin' and allow not requiring kotlin.stdlib in module-info (AnalysisFlags.allowKotlinPackage)"
     )
 
+    // It's inverted because otherwise we would have warnings in almost all KMP tests
+    val ENABLE_EXPECT_ACTUAL_CLASSES_WARNING by stringDirective(
+        description = "Disables -Xexpect-actual-classes key"
+    )
+
     // --------------------- Jvm Analysis Flags ---------------------
 
     val JVM_DEFAULT_MODE by enumDirective(