KT-29340 Provide effective modality for enum class in IR

* if enum class has abstract members, then it is ABSTRACT
* otherwise, if enum class has entries with members, then it is OPEN
* otherwise, it is FINAL.

compiler/testData/ir/irText/singletons/enumEntry.txt

@@ -1,5 +1,5 @@
 FILE fqName:<root> fileName:/enumEntry.kt
-  CLASS ENUM_CLASS name:Z modality:FINAL visibility:public flags: superTypes:[kotlin.Enum<Z>]
+  CLASS ENUM_CLASS name:Z modality:OPEN visibility:public flags: superTypes:[kotlin.Enum<Z>]
     $this: VALUE_PARAMETER INSTANCE_RECEIVER name:<this> type:Z flags:
     CONSTRUCTOR visibility:private <> () returnType:Z flags:primary
       BLOCK_BODY