[clangd] Improve documentation for auto and implicit specs

Summary:
Clangd didn't fill documentation for `auto` when it wasn't available in
index. Also it wasn't showing any documentations for implicit instantiations.

This patch ensures auto and normal decl case behaves in the same way and also
makes use of the explicit template specialization while fetching comments for
implicit instantiations.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71596