R1:409974fbd620

Add getDestinationPHIDs() to PhabricatorEdgeQuery

Summary: This should simplify a bunch of stuff in D3006 and D3003.

Test Plan: Will update D3006.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3010