[clangd] Use canonical declarations in ReferenceFinder.

Summary:
handleDeclOccurrencce reports a canonical declartion, so stick to use
canonical declarations to determine whether a declaration is in the
target set.

Also fix a previous ref test which misses a matched label (it fails without this
patch).

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D52871

llvm-svn: 343763