[clang][flang] MSVC-style options should warn instead of crash?

Passing MSVC-style options like `clang /DEFAULTLIB:foo ...` causes 'no such file or directory' errors with clang (and flang). Over in gh-119179, @tarunprabhu suggested that instead a warning should be emitted and such options should be ignored, similar to how clang-cl handles unknown clang-style options.

It's not clear to me how this would be implemented, though, since these options could instead be interpreted as absolute paths (starting with `/`)?