clang-cl: Add unsupported /Gv option (PR21892)

We don't currently support any of the calling convention options.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224199 91177308-0d34-0410-b5e6-96231b3b80d8
clang-cl: Make /Gd ignored instead of unsupported

/Gd is the default calling convention setting, so we don't
need to take any action.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224200 91177308-0d34-0410-b5e6-96231b3b80d8
clang-cl: ignore /cgthreads (PR21894)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224201 91177308-0d34-0410-b5e6-96231b3b80d8
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