[clangd] Improve output of semantic highlighting tests in case of failures

Summary:
Instead of matching lists of highlightings, we annotate input code with
resulting highlightings and diff it against the expected annotated input.

In case of failures, this produces much nicer output in form of text-based
diffs.

Reviewers: hokein

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67274

llvm-svn: 371373