[clang] [clang-tools-extra] [clang][Sema] Unify getPrototypeLoc helpers in SemaCodeComplete and clangd (PR #143345)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 approved this pull request.

thanks
Do we also want to add some tests?

https://github.com/llvm/llvm-project/pull/143345