[clangd] Fix crash in AddUsing tweak due to non-identifier DeclName

Patch by Adam Czachorowski!

Reviewers: hokein

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79582