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.
Before this commit, extension receivers were declared only for
properties with container source, which is strange & inconsistent.
Now we declare accessor extension receiver iff corresponding property
has extension receiver.
Since property accessor descriptors (unlike corresponding IR elements)
do not have type parameters, we need to take them from the corresponding
property to ensure the correct IR for delegated property accessors.
Juan Chen