Revert of Treat std::atomic_int as a trivial class member when checking for non-trivial classes.

tools/clang/plugins/tests/trivial_ctor.cpp

-// Copyright (c) 2016 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "trivial_ctor.h"
-
-// Due to https://bugs.chromium.org/p/chromium/issues/detail?id=663463, we treat
-// templated classes/structs as non-trivial, even if they really are trivial.
-// Thus, classes that have such a class/struct as a member get flagged as being
-// themselves non-trivial, even if (like |MySpinLock|) they are. Special-case
-// [std::]atomic_int.
-class TrivialTemplateOK {
- public:
-
- private:
-  MySpinLock lock_;
-};
-
-int main() {
-  TrivialTemplateOK one;
-  return 0;
-}