[clangd] Heuristically resolve dependent method calls

Summary:
The heuristic is to look in the definition of the primary template,
which is what you want in the vast majority of cases.

Fixes https://github.com/clangd/clangd/issues/141

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71240