[clang] [Frontend][Sema] Add CC1-only -fms-anonymous-structs to enable Microsoft anonymous struct/union feature (PR #176551)

[Hubert Tong via cfe-commits]

https://github.com/hubert-reinterpretcast approved this pull request.

LGTM with minor nits; however, we still have the open question of whether this should be a CC1-only option.

@MaskRay, do you have an opinion on whether this should be exposed via a Clang driver option (with or without a negative form)?

https://github.com/llvm/llvm-project/pull/176551