[clangd] Narrow rename to local symbols.

Summary:
Previously, we performed rename for all kinds of symbols (local, global).

This patch narrows the scope by only renaming symbols not being used
outside of the main file (with index asisitance). Renaming global
symbols is not supported at the moment (return an error).

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63426

llvm-svn: 364283