[clang] [clang] Support --sysroot= for ${arch}-windows-msvc targets (PR #96417)

[via cfe-commits]

https://github.com/zmodem commented:

I've read half the patch so far, but wanted to ask before continuing: would it be possible (and simpler) to extend the /winsysroot support somehow to also handle --sysroot?

(It would also be easier to review if the unrelated formatting changes were removed or split to a separate PR.)

+ @aganea and @mstorsjo in case you have comments too.

https://github.com/llvm/llvm-project/pull/96417