Make PrivacyPolicyBase covariant

Summary: Functions that return a `PrivacyPolicyBase` should have a return type of `this` that allows `getPrivacyPolicy` to be defined on Entish instead of an optional interface. This diff forces hack to think that `PrivacyPolicyBase` is covariant.

This diff is heavily based on D1287804.

Reviewed By: @jwatzman

Differential Revision: D1395604