Add a method to query whether an AASTNode is a descendent of another node

Summary: Ref T8674. This implementation is not really very efficient, but I'll try to iterate on that.

Test Plan: See next diff.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T8674

Differential Revision: https://secure.phabricator.com/D13819