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

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

Yeah, it seems some level of AI tools was used, at least according to https://github.com/matthiasgoergens/llvm-project/pull/5 . Please refer to our AI [policy](https://llvm.org/docs/AIToolPolicy.html)

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