[clang] [clang][Sema] Merge GNU diagnostic attributes by semantic spelling (PR #214523)

[Shengxin Pei via cfe-commits]

TPPPP72 wrote:

I think all these [PR](https://github.com/llvm/llvm-project/issues?q=is%3Apr+is%3Aopen+author%3Amatthiasgoergens) were generated by LLMs. Please read  [AI Tool Policy](https://llvm.org/docs/AIToolPolicy.html).

https://github.com/llvm/llvm-project/pull/214523