A Kotlin interface can have abstract members regardless of its modality. However, this invariant was previously ignored by FIR checkers. As a result, false-positive `ABSTRACT_<MEMBER>_IN_NON_ABSTRACT_CLASS` errors were being reported in explicitly non-abstract interfaces.
This commit makes a relevant FIR utility used by relevant FIR checkers aware of the aforementioned invariant.
#KT-66260 Fixed
Stanislav Ruban