[clangd] Document highlights for clangd

Summary: Implementation of Document Highlights Request as described in
LSP.

Contributed by William Enright (nebiroth).

Reviewers: malaperle, krasimir, bkramer, ilya-biryukov

Reviewed By: malaperle

Subscribers: mgrang, sammccall, klimek, ioeric, rwols, cfe-commits, arphaman, ilya-biryukov

Differential Revision: https://reviews.llvm.org/D38425

llvm-svn: 320474