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