Lint against undefined magic symbols

Summary: Fixes T5639. Allows the detection of undefined magic symbols, such as the use of __DIR__ in a codebase that targets < PHP 5.3.0.

Test Plan: Added a test case.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T5639

Differential Revision: https://secure.phabricator.com/D11386