[K/N] Mark WeakReference with ExperimentalNativeApi

As a part of efforts to stabilize Native stdlib #KT-55765.

kotlin-native/backend.native/tests/runtime/basic/cleaner_workers.kt

@@ -2,7 +2,7 @@
  * Copyright 2010-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
  * that can be found in the LICENSE file.
  */
-@file:OptIn(ExperimentalStdlibApi::class, FreezingIsDeprecated::class)
+@file:OptIn(ExperimentalStdlibApi::class, FreezingIsDeprecated::class, kotlin.experimental.ExperimentalNativeApi::class)
 
 package runtime.basic.cleaner_workers
 

kotlin-native/backend.native/tests/runtime/memory/cycles1.kt

@@ -3,6 +3,8 @@
  * that can be found in the LICENSE file.
  */
 
+@file:OptIn(kotlin.experimental.ExperimentalNativeApi::class)
+
 package runtime.memory.cycles1
 
 import kotlin.test.*

kotlin-native/backend.native/tests/runtime/memory/weak0.kt

@@ -3,6 +3,8 @@
  * that can be found in the LICENSE file.
  */
 
+@file:OptIn(kotlin.experimental.ExperimentalNativeApi::class)
+
 package runtime.memory.weak0
 
 import kotlin.test.*

kotlin-native/backend.native/tests/runtime/memory/weak1.kt

@@ -3,6 +3,8 @@
  * that can be found in the LICENSE file.
  */
 
+@file:OptIn(kotlin.experimental.ExperimentalNativeApi::class)
+
 package runtime.memory.weak1
 
 import kotlin.test.*

kotlin-native/backend.native/tests/runtime/workers/worker10.kt

@@ -1,4 +1,4 @@
-@file:OptIn(FreezingIsDeprecated::class)
+@file:OptIn(FreezingIsDeprecated::class, kotlin.experimental.ExperimentalNativeApi::class)
 package runtime.workers.worker10
 
 import kotlin.test.*

kotlin-native/runtime/src/main/kotlin/kotlin/native/ref/Weak.kt

@@ -5,11 +5,14 @@
 
 package kotlin.native.ref
 
+import kotlin.experimental.ExperimentalNativeApi
+
 /**
  * Class WeakReference encapsulates weak reference to an object, which could be used to either
  * retrieve a strong reference to an object, or return null, if object was already destroyed by
  * the memory manager.
  */
+@ExperimentalNativeApi
 public class WeakReference<T : Any> {
     /**
      * Creates a weak reference object pointing to an object. Weak reference doesn't prevent