Add a linter rule for abstract private methods

Summary:
Methods cannot be marked as both abstract and private. This language construct will cause a fatal error:

PHP Fatal error:  Abstract function X::Y() cannot be declared private in /home/josh/workspace/github.com/phacility/arcanist/test.php on line 4

Test Plan: Added unit tests.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14558