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