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

[Alexandre Ganea via cfe-commits]

aganea wrote:

@trcrsired Can you try https://crates.io/crates/xwin? (and use it with `--use-winsysroot-style`) This achieves the same effect as what you're doing here it seems. In practical terms, one can then do `clang-cl /winsysroot ...` as usual from Linux. Chromium does cross-compilation along the same lines (https://chromium.googlesource.com/chromium/src/+/lkgr/docs/win_cross.md)

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