Add a linter rule to prevent hardcoded class names

Summary: Add a linter rule to advise against the use of hardcoded class names. Hardcoded class names make the code harder to refactor and it is generally preferable to use the __CLASS__ magic constant instead.

Test Plan: This works, but needs some polish.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12605