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