[Syntax] Build SimpleDeclaration node that groups multiple declarators

Summary:
Also remove the temporary TopLevelDeclaration node and add
UnknownDeclaration to represent other unknown nodes.

See the follow-up change for building more top-level declarations.
Adding declarators is also pretty involved and will be done in another
follow-up patch.

Reviewers: gribozavr2

Reviewed By: gribozavr2

Subscribers: merge_guards_bot, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70787