Unify getPrototypeLoc functions in InlayHints.cpp and SemaCodeComplete.cpp

[HighCommander4]

We have two almost-identical utility functions, getPrototypeLoc() in clang-tools-extra/clangd/InlayHints.cpp and GetPrototypeLoc() in clang/lib/Sema/SemaCodeComplete.cpp.

To avoid code duplication and divergence in behaviour, we should unify them.

I think HeuristicResolver is a good place for a common implementation, as it fits the profile of what these functions are doing (trying to recover a FunctionProtoTypeLoc from a call expression on a best-effort basis).

[llvmbot]

@llvm/issue-subscribers-clangd

Author: Nathan Ridge (HighCommander4)

We have two almost-identical utility functions, [`getPrototypeLoc()`](https://searchfox.org/llvm/rev/269c29ae67e54bd3243535a7c58d0f30d1eb9036/clang-tools-extra/clangd/InlayHints.cpp#345) in `clang-tools-extra/clangd/InlayHints.cpp` and [`GetPrototypeLoc()`](https://searchfox.org/llvm/rev/269c29ae67e54bd3243535a7c58d0f30d1eb9036/clang/lib/Sema/SemaCodeComplete.cpp#6289) in `clang/lib/Sema/SemaCodeComplete.cpp`.

To avoid code duplication and divergence in behaviour, we should unify them.

I think HeuristicResolver is a good place for a common implementation, as it fits the profile of what these functions are doing (trying to recover a FunctionProtoTypeLoc from a call expression on a best-effort basis).

[HighCommander4]

Proposed patch: #143345