FIR/LC: fix modality of enum class

enum class, which is extended by enum entries, should not be final.
(Those enum entries should be, instead.)

enum class itself wouldn't be instantiated, and thus it's technically
abstract, but having a private ctor() would be an option.
For the latter case, we can simply put no modality modifier.

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/symbols/KtFirNamedClassOrObjectSymbol.kt

@@ -53,7 +53,13 @@ internal class KtFirNamedClassOrObjectSymbol(
     override val modality: Modality
         get() = getModality(
             FirResolvePhase.RAW_FIR,
-            if (classKind == KtClassKind.INTERFACE) Modality.ABSTRACT else Modality.FINAL
+            when (classKind) { // default modality
+                KtClassKind.INTERFACE -> Modality.ABSTRACT
+                // Enum class should not be `final`, since its entries extend it.
+                // It could be either `abstract` w/o ctor, or empty modality w/ private ctor.
+                KtClassKind.ENUM_CLASS -> Modality.OPEN
+                else -> Modality.FINAL
+            }
         )
 
     /* FirRegularClass visibility are not modified by STATUS only for Unknown so it can be taken from RAW */