[clangd] Match AST and Index label for template Symbols

Summary:
Previsouly, class completions items from the index were missing
template parameters in both the snippet and the label.

Reviewers: sammccall, hokein

Reviewed By: sammccall

Subscribers: klimek, jkorous-apple, ioeric, MaskRay, cfe-commits

Differential Revision: https://reviews.llvm.org/D45482

llvm-svn: 330004