[clangd] Enable indexing of template type parameters

Summary: Fixes https://bugs.llvm.org/show_bug.cgi?id=36285

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58294

llvm-svn: 354561