[clangd] Added highlightings for template parameters and specializations.

Summary: Template parameters and specializations were not being highlighted before. This adds highlightings to those types of tokens by adding two Visit* methods.

Reviewers: hokein, sammccall, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64855

llvm-svn: 366420