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

[via cfe-commits]

trcrsired wrote:

<img width="992" height="311" alt="image" src="https://github.com/user-attachments/assets/26fa8f66-94f8-4cdd-91b6-4c8e7c2b6b55" />
Even LLVM itself does POSIX style sysroot on windows for itself what a joke. The "wil" argument is completely flawed given cmake does exactly what POSIX style sysroot does.

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