[clangd] Report declaration references in findExplicitReferences.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68977

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@375226 91177308-0d34-0410-b5e6-96231b3b80d8