Move instantiability checks to the checking phase.

Summary: This enable us to use the `Classes` instead of the `ClassHeap`
hashtable for the instantiability check. The former is far smaller in
size since it does not include the ASTs. Deserialization is our most
expensive operation, so this improves performance significantly.

Reviewed By: @elgenie

Differential Revision: D1758703

Signature: t1:1758703:1419815450:b4b4c7c72b4fa75ef707737e01726c4dd1e0dae3