Native, K1: 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/util-klib-metadata/src/org/jetbrains/kotlin/library/metadata/impl/KlibResolvedModuleDescriptorsFactoryImpl.kt

@@ -8,6 +8,8 @@ package org.jetbrains.kotlin.library.metadata.impl
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.descriptors.*
+import org.jetbrains.kotlin.descriptors.annotations.AnnotationDescriptorImpl
+import org.jetbrains.kotlin.descriptors.annotations.Annotations
 import org.jetbrains.kotlin.descriptors.impl.ClassDescriptorImpl
 import org.jetbrains.kotlin.descriptors.impl.ModuleDescriptorImpl
 import org.jetbrains.kotlin.descriptors.impl.PackageFragmentDescriptorImpl
@@ -182,14 +184,26 @@ class ForwardDeclarationsPackageFragmentDescriptor(
         private val declarations = storageManager.createMemoizedFunction(this::createDeclaration)
 
         private val supertype by storageManager.createLazyValue {
-            val descriptor = builtIns.builtInsModule.getPackage(NativeStandardInteropNames.cInteropPackage)
+            findCinteropClass(supertypeName).defaultType
-                .memberScope
+        }
-                .getContributedClassifier(supertypeName, NoLookupLocation.FROM_BACKEND) as ClassDescriptor
 
-            descriptor.defaultType
+        private val experimentalAnnotationType by storageManager.createLazyValue {
+            findCinteropClass(NativeStandardInteropNames.ExperimentalForeignApi).defaultType
+        }
+
+        private fun findCinteropClass(name: Name): ClassDescriptor {
+            return builtIns.builtInsModule.getPackage(NativeStandardInteropNames.cInteropPackage)
+                .memberScope
+                .getContributedClassifier(name, NoLookupLocation.FROM_BACKEND) as ClassDescriptor
         }
 
         private fun createDeclaration(name: Name): ClassDescriptor {
+            val experimentalAnnotation = AnnotationDescriptorImpl(
+                experimentalAnnotationType,
+                emptyMap(),
+                SourceElement.NO_SOURCE
+            )
+
             return object : ClassDescriptorImpl(
                 this@ForwardDeclarationsPackageFragmentDescriptor,
                 name,
@@ -201,6 +215,7 @@ class ForwardDeclarationsPackageFragmentDescriptor(
                 LockBasedStorageManager.NO_LOCKS
             ) {
                 override fun isExpect(): Boolean = isExpect
+                override val annotations: Annotations = Annotations.create(listOf(experimentalAnnotation))
             }.apply {
                 this.initialize(MemberScope.Empty, emptySet(), null)
             }

core/compiler.common.native/src/org/jetbrains/kotlin/name/NativeStandardInteropNames.kt

@@ -12,6 +12,7 @@ object NativeStandardInteropNames {
     internal val CStructVar = Name.identifier("CStructVar")
     internal val ObjCObjectBase = Name.identifier("ObjCObjectBase")
     internal val ObjCObject = Name.identifier("ObjCObject")
+    val ExperimentalForeignApi = Name.identifier("ExperimentalForeignApi")
 
     object ForwardDeclarations {
         private val cNamesPackage = FqName("cnames")