[clangd] Add highlighting for macro expansions.

Summary: https://github.com/clangd/clangd/issues/134

Reviewers: hokein, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66995

llvm-svn: 370482