[clangd] Show better message when we rename macros.

Summary:
Previously, when we rename a macro, we get an error message of "there is
no symbol found".

This patch improves the message of this case (as we don't support macros).

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63922

llvm-svn: 364735