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

[Martin Storsjö via cfe-commits]

mstorsjo 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 for all the casing issues and supports the /winsysroot layout if asked.

FWIW, http://github.com/mstorsjo/msvc-wine/ also provides a non-rust solution to the same, which also provides a lowercased/symlink forest variant of includes/libraries.

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