[clangd] Tweak the no-index error message for rename, NFC.

Summary: The current error message doesn't fit well for cross-file rename.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70809