[clangd] Fix an assertion crash in ReferenceFinder.

Summary: The assertion is almost correct, but it fails on refs from non-preamble

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77222