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

[Hans Wennborg via cfe-commits]

zmodem wrote:

> I've always kind of held out this idea that clang[++] are perfectly reasonable frontends to use when targeting an MSVC-environment with an official Microsoft Windows SDK

Me too, but isn't this PR a different idea? This is about using clang with MSVC/Win SDK turned into a Unix-style sysroot, which seems like a much narrower use case.

I think it would be  better if the driver could work with the MSVC/SDK as-is, rather than forcing the user to change the SDK into a form that the driver likes (even if it's a "standard" form in the sense that other sysroots use it). That would probably require more work, such as supporting case-insensitive includes, but seems like it would be much user friendlier.

But I won't block this if there's otherwise strong support for it.

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