"abstract" modifier on enum class is an error

#KT-3454 Fixed

compiler/testData/diagnostics/tests/controlFlowAnalysis/UninitializedOrReassignedVariables.kt

@@ -68,7 +68,7 @@ fun t1() {
     b = <!UNUSED_VALUE!>3<!>
 }
 
-abstract enum class ProtocolState {
+enum class ProtocolState {
   WAITING {
     override fun signal() = ProtocolState.TALKING
   }

compiler/testData/diagnostics/tests/controlFlowAnalysis/UninitializedOrReassignedVariables.txt

@@ -116,7 +116,7 @@ package uninitialized_reassigned_variables {
         }
     }
 
-    internal abstract enum class ProtocolState : kotlin.Enum<uninitialized_reassigned_variables.ProtocolState> {
+    internal final enum class ProtocolState : kotlin.Enum<uninitialized_reassigned_variables.ProtocolState> {
         private constructor ProtocolState()
         public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
         public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int