[clangd] Print template arguments helper

Summary:
Prepares ground for printing template arguments as written in the
source code, part of re-landing rC356541 with D59599 applied.

Reviewers: ioeric, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59639

llvm-svn: 358272