Add a `getEndLineNumber` method to the `AASTNode` class

Summary: Adds a `getEndLineNumber` method to the `AASTNode` class. This method supplements the `getLineNumber` method, providing the //ending// line number for a given node.  This feels a little awkward, but it makes D10535 a little nicer.

Test Plan: See D10535.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10534