[clangd] Added semantic highlighting support for primitives.

[clangd] Added semantic highlighting support for primitives.

Summary:
Adds a new HighlightingKind "Primitive". Adds a special case for TypeLocs that have an underlying TypePtr that is are builtin types, adding them as primitives.
The primary reason for this change is because otherwise typedefs that typedef primitives `typedef int A` would not get highlighted (so in the example `A` would not get any highlightings.)

Reviewers: hokein, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65943

llvm-svn: 368291