Fir2Ir: more precise calculation of enum class modality

Use the same condition as in the already existing `createIrEnumEntry`
function (and as in psi2ir): enum class should be final unless there's
an enum entry with any declaration other than its constructor.

 #KT-57216

compiler/testData/ir/irText/firProblems/thisInEnumConstructor.fir.kt.txt

@@ -1,4 +1,4 @@
-open enum class EE : Enum<EE> {
+enum class EE : Enum<EE> {
   private constructor(myName: String = Companion.toString().lowercase()) /* primary */ {
     super/*Enum*/<EE>()
     /* <init>() */