[clangd] Add a hidden tweak to annotate all highlighting tokens of the file.

Reviewers: sammccall, jvikstrom

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64137

llvm-svn: 365130