[Syntax] Build nodes for simple cases of top level declarations

Summary:
More complicated nodes (e.g. template declarations) will be implemented
in the follow-up patches.

Reviewers: gribozavr2

Reviewed By: gribozavr2

Subscribers: merge_guards_bot, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70856