NFC: unify clang / LLVM atomic ordering

Summary:
Depends on http://reviews.llvm.org/D18875

This makes the C11 / C++11 *ABI* atomic ordering accessible from LLVM, as discussed in http://reviews.llvm.org/D18200#inline-151433

Reviewers: jyknight, reames

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18876

llvm-svn: 266574