NFC: unify clang / LLVM atomic ordering

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

This re-applies r266573 which I had reverted in r266576.

Original review: http://reviews.llvm.org/D18875

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266640 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed