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