Use __CLANG_ATOMIC_TYPE_LOCK_FREE macros in `stdatomic.h`

Summary: This patch makes the header `stdatomic.h` work when `-fms-compatibility` is specified.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D32322

llvm-svn: 300919