IR: minor, make IrClass.companionObject return IrClass

(cherry picked from commit c2f34c96f96fe52b3f3fbbe5de0313d7712f67f6)

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/lower/InteropCallConvertors.kt

@@ -229,7 +229,7 @@ private fun InteropCallContext.convertIntegralToEnum(
         enumType: IrType
 ): IrExpression {
     val enumClass = enumType.getClass()!!
-    val companionClass = enumClass.companionObject()!! as IrClass
+    val companionClass = enumClass.companionObject()!!
     val byValue = companionClass.simpleFunctions().single { it.name.asString() == "byValue" }
     val byValueArg = castPrimitiveIfNeeded(value, intergralType, byValue.valueParameters.first().type)
     return builder.irCall(byValue).apply {
@@ -474,4 +474,4 @@ private fun InteropCallContext.generateBitFieldAccess(callSite: IrCall): IrExpre
         }
         else -> error("Unexpected accessor function: ${accessor.name}")
     }
-}
+}