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.
* If `toArray` is inherited from Java, it may take an argument or
return a value of a flexible type, which looks nullable in IR;
* The returned array may also have `out` variance of the type
argument;
* `Array<T>` is not the same as `Array<Any?>` if `T` is neither `Any`
nor `in Something`, so presence of `toArray(): Array<T>` does not
mean we don't need to generate a new `toArray`.
Denis Zharkov