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

[via cfe-commits]

trcrsired wrote:

> > Also cargo-xwin has picked my solution of windows-msvc-sysroot. It is proven to be a good solution https://github.com/rust-cross/cargo-xwin
> 
> Also, I doubt that they have chosen to use it because they think the layout is in any way better; they probably do it because it is a quicker and simpler way of getting the MSVC libraries and the WinSDK, not realizing that it is in violation of the license.

Tell me why clang --sysroot should not follow GNU style sysroot when clang itself literally aligns the behavior of gcc? Also how do you support libc++ when visual studio puts C library and MS STL headers in the same dir?

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