[clangd] Simplify semantic highlighting visitor

Summary:
- Functions to compute highlighting kinds for things are separated from
  the ones that add highlighting tokens.
  This keeps each of them more focused on what they're doing: getting
  locations and figuring out the kind of the entity, correspondingly.

- Less special cases in visitor for various nodes.

This change is an NFC.

Reviewers: hokein

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67341

llvm-svn: 372008