[clang] [C23] Implement WG14 N3037 (PR #132939)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> So one usability issue I noticed that we also had in GCC is that the warning about declaring structs in function prototypes (-Wvisibility) does not make sense anymore and becomes annoying. We turned it off in C23 for complete (but not for incomplete) structs) without tag.

https://github.com/llvm/llvm-project/pull/138516

https://github.com/llvm/llvm-project/pull/132939