[clangd] Replace HighlightingKind::NumKinds with LastKind. NFC

Summary:
The latter simplifies the client code by avoiding the need to handle it
as a separate case statement.

Reviewers: hokein

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67277

llvm-svn: 371375