[clangd] Replace raw lexer code with token buffer in prepare rename.

Summary:
there is a slight behavior change in this patch:

- before: `in^t a;`, returns our internal error message (no symbol at given location)
- after: `in^t a, returns null, and client displays their message (e.g.
   e.g. "the element can't be renamed" in vscode).

both are sensible according LSP, and we'd save one `rename` call in the later case.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73610