[clangd][Hover] Handle uninstantiated templates

Summary: Fixes https://github.com/clangd/clangd/issues/263

Reviewers: hokein, sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73344