[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