[Syntax] Build template declaration nodes

Summary:
Copy of https://reviews.llvm.org/D72334, submitting with Ilya's permission.

Handles template declaration of all kinds.

Also builds template declaration nodes for specializations and explicit
instantiations of classes.

Some missing things will be addressed in the follow-up patches:

specializations of functions and variables,
template parameters.

Reviewers: gribozavr2

Reviewed By: gribozavr2

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76346