Add a linter rule for use of is_a

Summary: instanceof should be used instead of is_a. I need to do a bit more research here to see if there are any edge cases.

Test Plan: Added unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14573