[clangd] Added highlighting for variable references (declrefs)

Summary: Added highlighting for variable references using VisitDeclRefExpr.

Reviewers: hokein, sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64199

llvm-svn: 365205