Make FreezableAtomicReference atomic when unfrozen for the new MM.

kotlin-native/runtime/src/mm/cpp/Memory.cpp

@@ -82,7 +82,8 @@ void ObjHeader::destroyMetaObject(ObjHeader* object) {
 }
 
 ALWAYS_INLINE bool isPermanentOrFrozen(const ObjHeader* obj) {
-    return mm::IsFrozen(obj);
+    // TODO: Freeze TF_IMMUTABLE objects upon creation.
+    return mm::IsFrozen(obj) || ((obj->type_info()->flags_ & TF_IMMUTABLE) != 0);
 }
 
 ALWAYS_INLINE bool isShareable(const ObjHeader* obj) {