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

[via cfe-commits]

zmodem wrote:

> no. I do not want that complicated file structure. I just want normal GNU style's sysroot.
>
> x86_64-windows-msvc sysroot should have exact the same file struct as x86_64-windows-gnu.

The -msvc triples are designed to work with libraries from MSVC and the Windows SDK. We can't change how those are organized.

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