[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