[C++11] Use std::atomic instead of LLVM's.

No intended functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202652 91177308-0d34-0410-b5e6-96231b3b80d8
[C++11] ASTMatchers: Use standard static_assert and type traits.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202653 91177308-0d34-0410-b5e6-96231b3b80d8