Provide more buildXClause() and buildXClauseParts() on PolicyAwareQuery

Summary:
Ref T4100. Ref T5595. These functions are trivial for now, but move us toward being able to define more default query behavior by default.

Future changes will give these methods meaningful, nontrivial behaviors.

Test Plan: arc unit --everything

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5595, T4100

Differential Revision: https://secure.phabricator.com/D12454