The /volatile:ms semantics turn volatile loads and stores into atomic
acquire and release operations.  This distinction is important because
volatile memory operations do not form a happens-before relationship
with non-atomic memory.  This means that a volatile store is not
sufficient for implementing a mutex unlock routine.

Differential Revision: http://reviews.llvm.org/D7580

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@229082 91177308-0d34-0410-b5e6-96231b3b80d8