[clangd] Do not highlight keywords in semantic highlighting

Summary:
Editors are good at highlightings the keywords themselves.
Note that this only affects highlightings of builtin types spelled out
as keywords in the source code. Highlightings of typedefs to builtin
types are unchanged.

Reviewers: hokein

Reviewed By: hokein

Subscribers: merge_guards_bot, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69431