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 r266574 which I had reverted in r266575.

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

Original review: http://reviews.llvm.org/D18876

llvm-svn: 266641