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

[Riyaz Ahmad via cfe-commits]

riyaz86a 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.

@hubert-reinterpretcast 
I just added release note for this option, will remove it if it's not required.
New CC1 option ``-fms-anonymous-structs`` added to enable only Microsoft's anonymous struct/union extension without enabling other ``-fms-extensions`` features [GH177607].

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