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

[via cfe-commits]

zmodem wrote:

> I extract all files from msvc installations and windows SDK and put them in the format of what a normal sysroot would do on other platforms. Basically, unify the behavior with another platform.

Isn't it better to point Clang at MSVC and the Windows SDK as they're normally installed?

This patch seems specific to your custom setup, not something that would be useful for others.

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