Add a few methods to `AASTNode` and `AASTToken`

Summary: Add a few general purpose methods to the `AASTNode` and `AASTToken` class.

Test Plan: See D10558.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10554