Add a linter rule for invalid default parameters

Summary: See http://www.phpwtf.org/default-arguments-and-type-hinting.

Test Plan: Added test case.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12855