This change is necessary in order to make `FirElement` abstract class. To do it we need to remove all `accept` and `transform` methods from fir interfaces. So we have to make sure that all fir element implementations have their `accept` and `transform` implementations in their super classes. In particular this change makes `FirImport` abstract class in order to preserve `accept` and `transform` methods after removing them from fir interfaces.
Arseniy Terekhov
4d85cc8aa6