[clangd] Don't perform rename when the refs result from index is incomplete.

Summary:
Also do an early return if the number of affected files > limit to save
some unnecessary FileURI computations.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70811