[clangd] Highlight related control flow.

Summary:
This means e.g. highlighting "return" will show other returns/throws
from the same function, highlighting a case will show all the
return/breaks etc.

This is a bit of an abuse of textDocument/highlight, but seems useful.

Reviewers: adamcz

Subscribers: ilya-biryukov, MaskRay, jkorous, mgrang, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78454