[clangd] Duplicate lines of semantic highlightings sent removed.

Summary: Added a class for diffing highlightings and removing duplicate lines. Integrated into the highlighting generation flow. Only works correctly if all tokens are on a single line. Also returns empty lines if the IDE should remove previous highlightings on a line.

Reviewers: hokein, sammccall, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64475

llvm-svn: 367521