Support some QueryConstraint operations against generic ApplicationSearch query logic

Summary: Ref T13090. Currently, it isn't possible to query custom fields for complex constraints. Lay the groundwork to implement some of the easy ones (none(), any()) for Datasource/PHID fields.

Test Plan: Hard-coded some constraints and queried with them; see next change for more substantial testing.

Maniphest Tasks: T13090

Differential Revision: https://secure.phabricator.com/D19126