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