clang-cl: Fix handling of the /volatile flag (PR21893)

The /volatile:iso flag is our default behaviour, so it can be ignored.

Parse /volatile:ms as unsupported.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224202 91177308-0d34-0410-b5e6-96231b3b80d8