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

[Alexandre Ganea via cfe-commits]

aganea wrote:

We don’t have to do anything in practice, [Xwin](https://github.com/Jake-Shadle/xwin) is already such a « script » and it already creates symlinks and supports the /winsysroot layout if asked.

What is the advantage of using clang (not CL)? We’ve been telling users for years to not do that? —sysroot could just use the /winsysroot layout.

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