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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> This change needs a release note.

@cor3ntin, the current scope only adds a CC1 option. That does not warrant a release note, does it?
We'll add a release note if this ends up being a Clang driver option.

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