[clangd] Capture the missing injected class names in findExplicitReferences.

Summary: Fixes https://github.com/clangd/clangd/issues/237.

Reviewers: kadircet, kbobyrev

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73088