[clangd] Move Rename into its own file, and add unit test. NFC

Reviewers: kadircet

Subscribers: mgorny, ilya-biryukov, MaskRay, jkorous, arphaman, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61596

llvm-svn: 360116