Remove mostly-unused PreClass* field from PreClass::{Prop,Const}

Summary: It was only being used in their pretty-print methods, but those are only
called from PreClass::prettyPrint, so we can just pass the current PreClass*
over to them instead of having to store it.

Reviewed By: @bertmaher

Differential Revision: D1681944

Signature: t1:1681944:1415999560:911757507ea045d3c4e9cf0269aa9221b1f9ed8d