[clang] [clang][Driver] Support -fms-volatile as equivalent to /volatile:ms (PR #74790)

[Reid Kleckner via cfe-commits]

https://github.com/rnk commented:

> I assume that /volatile:ms is a legacy behavior that non-Windows OSes likely don't want to adopt.

That is correct, it is legacy behavior. However, I think this flag has potential porting applications, similar to `-fshort-wchar`. I also think operating systems don't really have a say or control over what command line flags developers use. I think the more relevant question is, does the Clang project want to encourage folks to use this flag, thereby increasing our long term support burden? If this is a legacy compatibility mode, do we think we will be able to sunset it at some point, similar to `-fdelayed-template-parsing`?

https://github.com/llvm/llvm-project/pull/74790