[clangd] findExplicitReferences supports goto labels

Summary:
This means they're renamable and textDocument/highlight works

This fell out of D78454

Reviewers: adamcz

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80170