Native, K2: add ExperimentalForeignApi to generated forward declarations

cinterop tool should add ExperimentalForeignApi to all generated
declarations, to indicate their experimental status and discourage using
them in public Kotlin API. But the same considerations are applicable
to forward declarations (cnames.*, objcnames.*), which are generated not
by cinterop tool, but directly by the compiler.

This commit adds ExperimentalForeignApi to those compiler-generated
classes.

^KT-58362

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/session/NativeForwardDeclarationsSymbolProvider.kt

@@ -18,6 +18,8 @@ import org.jetbrains.kotlin.fir.declarations.FirResolvePhase
 import org.jetbrains.kotlin.fir.declarations.builder.buildRegularClass
 import org.jetbrains.kotlin.fir.declarations.getDeprecationsProvider
 import org.jetbrains.kotlin.fir.declarations.impl.FirResolvedDeclarationStatusImpl
+import org.jetbrains.kotlin.fir.expressions.builder.buildAnnotation
+import org.jetbrains.kotlin.fir.expressions.impl.FirEmptyAnnotationArgumentMapping
 import org.jetbrains.kotlin.fir.resolve.providers.FirSymbolNamesProvider
 import org.jetbrains.kotlin.fir.resolve.providers.FirSymbolNamesProviderWithoutCallables
 import org.jetbrains.kotlin.fir.resolve.providers.FirSymbolProvider
@@ -26,13 +28,15 @@ import org.jetbrains.kotlin.fir.scopes.FirKotlinScopeProvider
 import org.jetbrains.kotlin.fir.symbols.impl.*
 import org.jetbrains.kotlin.fir.types.builder.buildResolvedTypeRef
 import org.jetbrains.kotlin.fir.types.constructClassType
+import org.jetbrains.kotlin.fir.types.toLookupTag
 import org.jetbrains.kotlin.library.KotlinLibrary
 import org.jetbrains.kotlin.library.includedForwardDeclarations
 import org.jetbrains.kotlin.library.isInterop
-import org.jetbrains.kotlin.name.NativeForwardDeclarationKind
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
+import org.jetbrains.kotlin.name.NativeForwardDeclarationKind
+import org.jetbrains.kotlin.name.NativeStandardInteropNames
 
 class NativeForwardDeclarationsSymbolProvider(
     session: FirSession,
@@ -113,6 +117,18 @@ class NativeForwardDeclarationsSymbolProvider(
                 type = ConeClassLikeLookupTagImpl(forwardDeclarationKind.superClassId)
                     .constructClassType(emptyArray(), isNullable = false)
             }
+
+            annotations += buildAnnotation {
+                annotationTypeRef = buildResolvedTypeRef {
+                    val annotationClassId = ClassId(
+                        NativeStandardInteropNames.cInteropPackage,
+                        NativeStandardInteropNames.ExperimentalForeignApi
+                    )
+                    type = annotationClassId.toLookupTag()
+                        .constructClassType(typeArguments = emptyArray(), isNullable = false)
+                }
+                argumentMapping = FirEmptyAnnotationArgumentMapping
+            }
         }.apply {
             replaceDeprecationsProvider(getDeprecationsProvider(session))
         }