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

[via cfe-commits]

YexuanXiao wrote:

@rnk @sharadhr I want to clarify that this patch does not replace clang-cl. As an MSVC user myself, I know that many projects only support MSVC-style command-line flags, which is why clang-cl remains the best option for compatibility with MSVC. This patch adds support for the Clang-style command-line interface. Besides facilitating cross-compilation, this patch also enables libc++ to work and allows debugging with Visual Studio, so it serves a different purpose compared to MinGW.

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