[clangd] Emit better error messages when rename fails.

Summary:
Currently we emit an unfriendly "clang diagnostic" message when rename fails. This
patch makes clangd to emit a detailed diagnostic message.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60821

llvm-svn: 358658