Add a getTree() method to AASTNode
Summary: Basically, I want to use AASTNodeList::newFromTreeAndNodes, but don't have access to the tree.
Test Plan: N/A
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D11781