[FIR][checkers] Respect ClassKind.INTERFACE in FirRegularClass.canHaveAbstractDeclaration

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

compiler/testData/diagnostics/tests/modifiers/OpenInterface.fir.txt

@@ -1,4 +1,4 @@
-FILE: OpenInterface.fir.kt
+FILE: OpenInterface.kt
     public open interface OpenInterface : R|kotlin/Any| {
         public abstract fun defaultModalityFuncWithoutBody(): R|kotlin/Unit|