Move mirror::Class to use ObjPtr

Leave the return types as non ObjPtr for now. Fixed moving GC bugs
in tests.

Test: test-art-host

Bug: 31113334

Change-Id: I5da1b5ac55dfbc5cc97a64be2c870ba9f512d9b0
diff --git a/runtime/class_linker-inl.h b/runtime/class_linker-inl.h
index fa971c4..dba9b8f 100644
--- a/runtime/class_linker-inl.h
+++ b/runtime/class_linker-inl.h
@@ -200,7 +200,7 @@
 inline mirror::Object* ClassLinker::AllocObject(Thread* self) {
   return GetClassRoot(kJavaLangObject)->Alloc<true, false>(
       self,
-      Runtime::Current()->GetHeap()->GetCurrentAllocator());
+      Runtime::Current()->GetHeap()->GetCurrentAllocator()).Ptr();
 }
 
 template <class T>