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