[Syntax] Build template declaration nodes

Summary:
Rollforward of
https://reviews.llvm.org/rGdd12826808f9079e164b82e64b0697a077379241 after
temporarily adding -fno-delayed-template-parsing to the TreeTest.

Original 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

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76418