This commit handles "subclass: super-interface by delegate-expression".
During Psi2Fir, for each delegate, we add to the subclass a synthetic
field (which has type super-interface), and an assignment of the
delegate-expression to the synthetic field in the primary constructor,
so that the delegate-expression can be resolved and transformed along
the way.
During Fir2Ir, we look up delegatable members from the super-interface
and generate corresponding functions/properties for the subclass.
TODO: support for generic delegatable members and generic
super-interface.
Juan Chen