Add an `selectDescendantsOfTypes` to the `AASTNode`.

Summary: This method generally seems to make sense (there already exists such a method in the `AASTNodeList` class). Additionally, such a method simplifies D9117 slightly.

Test Plan: Ran the unit tests in rARC, which generally rely on this class.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9134