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/classes/enum.fir.kt.txt

@@ -18,7 +18,7 @@ enum class TestEnum1 : Enum<TestEnum1> {
 
 }
 
-open enum class TestEnum2 : Enum<TestEnum2> {
+enum class TestEnum2 : Enum<TestEnum2> {
   private constructor(x: Int) /* primary */ {
     super/*Enum*/<TestEnum2>()
     /* <init>() */
@@ -131,7 +131,7 @@ abstract enum class TestEnum4 : Enum<TestEnum4> {
 
 }
 
-open enum class TestEnum5 : Enum<TestEnum5> {
+enum class TestEnum5 : Enum<TestEnum5> {
   private constructor(x: Int = 0) /* primary */ {
     super/*Enum*/<TestEnum5>()
     /* <init>() */
@@ -161,7 +161,7 @@ fun f(): Int {
   return 1
 }
 
-open enum class TestEnum6 : Enum<TestEnum6> {
+enum class TestEnum6 : Enum<TestEnum6> {
   private constructor(x: Int, y: Int) /* primary */ {
     super/*Enum*/<TestEnum6>()
     /* <init>() */