[clangd] Enable cross-file-rename by default.

Summary:
The cross-file rename feature is stable enough to enable it (has been
rolled out internally for a few weeks).

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80507