Add a getLineNumber() method to AASTToken

Summary: Adds an AASTToken::getLineNumber() method, similar to AASTNode::getLineNumber().

Test Plan: See D13534.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13889