[clangd] Clean-up XRefs.cpp from Lexer usages and unnecessary SourceLoc transformations

Summary:
Get rid of calls to lexer and unnecessary source location
transformations.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75166