[clangd] Add a distinct highlighting for local variables

Summary:
It's useful to be able to distinguish local variables from namespace
scope variables.

Reviewers: hokein, jvikstrom

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66723

llvm-svn: 370103