[clangd] Show template arguments in type hierarchy when possible

Summary: Fixes https://github.com/clangd/clangd/issues/31

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71533