[k1/K2]: Mark @SubclassOptInRequired as an experimental

At the moment, SubclassOptInRequired is marked with the
ExperimentalSubclassOptIn annotation. However, it does not work
as expected due to a missing opt-in error. To use SubclassOptInRequired,
an explicit opt-in is necessary because SubclassOptInRequired is an
unstable feature now.

^KT-64739

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/CorrectApplicability.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 @RequiresOptIn
 annotation class ApiMarker

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/Experimentality.kt

@@ -0,0 +1,7 @@
+// FIR_IDENTICAL
+
+@RequiresOptIn
+annotation class ApiMarker
+
+@<!OPT_IN_USAGE_ERROR!>SubclassOptInRequired<!>(ApiMarker::class)
+open class OpenKlass

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/IncorrectApplicability.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 @RequiresOptIn
 annotation class ApiMarker

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/InheritingByLocalClassifiers.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 @RequiresOptIn
 annotation class ApiMarker

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/InheritingDifferentOptInLevels.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 @RequiresOptIn
 annotation class ApiMarker

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/InnerAndNestedClasses.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 @RequiresOptIn
 annotation class Api

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/InnerClasses.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 @RequiresOptIn
 annotation class Boom

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/NestedClasses.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 @RequiresOptIn
 annotation class Boom

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/NotApiMarkerAsArgument.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 annotation class DummyAnnotation
 

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/Typealias.fir.kt

@@ -1,3 +1,5 @@
+@file:OptIn(ExperimentalSubclassOptIn::class)
+
 @RequiresOptIn
 annotation class ApiMarker
 

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/Typealias.kt

@@ -1,3 +1,5 @@
+@file:OptIn(ExperimentalSubclassOptIn::class)
+
 @RequiresOptIn
 annotation class ApiMarker
 

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/UsageOptInIsNotImplied.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 @RequiresOptIn
 annotation class ApiMarker

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/WithAbstractClasses.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 @RequiresOptIn
 annotation class ApiMarker

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/WithInheritanceByDelegation.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 @RequiresOptIn
 annotation class ApiMarker

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/WithInterfaces.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 @RequiresOptIn
 annotation class ApiMarker

compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/WithOpenClasses.kt

@@ -1,4 +1,5 @@
 // FIR_IDENTICAL
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 @RequiresOptIn
 annotation class ApiMarker