[clangd][ObjC] Improve xrefs for protocols and classes

Summary:
Previously clangd would jump to forward declarations for protocols
and classes instead of their definition/implementation.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83501