JVM IR: Create package private synthetic multi file part classes

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/ir/IrUtils.kt

@@ -393,7 +393,7 @@ val IrDeclaration.isTopLevel: Boolean
     get() {
         if (parent is IrPackageFragment) return true
         val parentClass = parent as? IrClass
-        return parentClass?.origin == IrDeclarationOrigin.FILE_CLASS && parentClass.parent is IrPackageFragment
+        return parentClass?.isFileClass == true && parentClass.parent is IrPackageFragment
     }
 
 fun Scope.createTemporaryVariableWithWrappedDescriptor(

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/SingleAbstractMethodLowering.kt

@@ -65,7 +65,7 @@ abstract class SingleAbstractMethodLowering(val context: CommonBackendContext) :
     abstract fun getSuperTypeForWrapper(typeOperand: IrType): IrType
 
     override fun lower(irFile: IrFile) {
-        enclosingContainer = irFile.declarations.filterIsInstance<IrClass>().find { it.origin == IrDeclarationOrigin.FILE_CLASS }
+        enclosingContainer = irFile.declarations.filterIsInstance<IrClass>().find { it.isFileClass }
             ?: irFile
         irFile.transformChildrenVoid()