[clang] [Clang] Support multilibs on Windows (PR #200212)

[Yaxun Liu via cfe-commits]

https://github.com/yxsamliu approved this pull request.

LGTM. This wires MSVC into the existing multilib config flow and now covers both include and library search paths. The added tests cover selection, layering, printing, and error handling. My remaining release-note wording comment is non-blocking.

https://github.com/llvm/llvm-project/pull/200212