Before this commit we initialized delegate fields in primary constructor,
that could provoke NPE in case delegate is used in initializer of
some property backing field.
Now we initialize delegate fields directly instead.
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.
Denis.Zharkov