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

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> Do we also want to add some tests?

Yeah, good point. The two separate implementations did have test coverage in their respective components (CodeComplete and clangd), but `HeuristicResolver` has its own test suite so it would be better to have some tests for it there as well. Will revise the patch to add some.

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