Simplify the implementation of PhabricatorPHIDType subclasses
Summary: Instead of implementing the getTypeConstant method in all subclasses of PhabricatorPHIDType, provide a final implementation in the base class which uses reflection. See D9837 for a similar implementation.
Test Plan: Ran arc unit.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: epriestley, Korvin, hach-que
Differential Revision: https://secure.phabricator.com/D9985