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

[via cfe-commits]

trcrsired wrote:

> 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.

no. i do not use clang-cl. /winsysroot is not sysroot. /winsysroot is for clang-cl. not clang itself

sysroot is what the usually the clang and GCC means. That is not a semantics for msvc.

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