[clangd] Speed up when building rename edit.

Summary:
We used to scan the code everytime when computing the LSP position to the offset
(respect the LSP encoding). Now we only scan the source code once.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70441