[RFC] Add support for controlling diagnostics severities at file-level granularity through command line

Raising a question by @AaronBallman here;

should this be exposed in clang-cl as well? What about flang? CC @zmodem @rnk @MaskRay @jansvoboda11 for opinions

I believe exposing this for clang-cl totally makes sense, but I need to verify if path-handling works fine with windows backslashes (note that any issues here do already affect sanitizer specialcaselists on windows).

What about flang?

I am not too familiar with flang, if people think it’d be useful and can add some tests covering flang warnings, I don’t see any reason not to.