[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/fir/tree/src/org/jetbrains/kotlin/fir/declarations/utils/FirStatusUtils.kt

@@ -62,7 +62,7 @@ inline val FirClass.isEnumClass: Boolean
 inline val FirRegularClass.isSealed: Boolean get() = status.modality == Modality.SEALED
 
 inline val FirRegularClass.canHaveAbstractDeclaration: Boolean
-    get() = isAbstract || isSealed || isEnumClass
+    get() = isInterface || isAbstract || isSealed || isEnumClass
 
 inline val FirRegularClass.isCompanion: Boolean get() = status.isCompanion
 inline val FirRegularClass.isData: Boolean get() = status.isData

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

@@ -1,15 +0,0 @@
-// FIR_DUMP
-<!REDUNDANT_MODIFIER_FOR_TARGET!>open<!> interface OpenInterface {
-    fun <!ABSTRACT_FUNCTION_IN_NON_ABSTRACT_CLASS!>defaultModalityFuncWithoutBody<!>()
-    fun defaultModalityFuncWithBody() {}
-    <!ABSTRACT_FUNCTION_IN_NON_ABSTRACT_CLASS!>abstract<!> fun abstractFunc()
-    open fun openFunc() {}
-
-    val <!ABSTRACT_PROPERTY_IN_NON_ABSTRACT_CLASS!>defaultModalityValWithoutGetter<!>: Any
-    val defaultModalityValWithGetter: Any get() = 42
-    <!ABSTRACT_PROPERTY_IN_NON_ABSTRACT_CLASS!>abstract<!> val abstractVal: Any
-    open val openVal: Any get() = 42
-
-    <!ABSTRACT_PROPERTY_IN_NON_ABSTRACT_CLASS!>abstract<!> val abstractValWithGetter: Any get() = 42
-    <!ABSTRACT_PROPERTY_IN_NON_ABSTRACT_CLASS!>abstract<!> var abstractValWithSetter: Any set(value) {}
-}

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|
 

compiler/testData/diagnostics/tests/modifiers/OpenInterface.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // FIR_DUMP
 <!REDUNDANT_MODIFIER_FOR_TARGET!>open<!> interface OpenInterface {
     fun defaultModalityFuncWithoutBody()