[clangd] Implement cross-file rename.

Summary:
This is the initial version. The cross-file rename is purely based on
the index.

It is hidden under a command-line flag, and only available for a small set
of symbols.

Reviewers: ilya-biryukov, sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69263