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

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

Updated patch to add dedicated `HeuristicResolver` tests. And I did catch a bug in the unified implementation in the process, so thank you for suggesting it:)

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