[include-cleaner] Improve handling for templates

Principal here is:
- Making sure each template instantiation implies use of the most specialized
  template. As explicit instantiations/specializations are not redeclarations of
  the primary template.
- Introducing a use from explicit instantions/specializaitons to the primary
  template, as they're required but not traversed as part of the RAV.

Differential Revision: https://reviews.llvm.org/D148112