[clangd] Implement a function to lex the file to find candidate occurrences.

Summary:
This will be used for incoming cross-file rename (to detect index
staleness issue).

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69615