[clangd] Navigation from definition of template specialization to primary template

Fixes https://github.com/clangd/clangd/issues/212.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71090