CodeGen: _Atomic(_Complex) shouldn't crash

We could be a little kinder if we did a compare-exchange loop instead of
an atomic-load/store pair.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@229212 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed