[clangd] Avoid crashes in override completions

Summary: NamedDecl::getName cannot be called on non-identifier names.

Reviewers: kadircet, ioeric, hokein, sammccall

Reviewed By: ioeric

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D51598

llvm-svn: 341322