Fix abstract nested class in enums in JVM back-end

It caused InstantiationError because the enum had an incorrect ACC_ABSTRACT
flag

compiler/testData/codegen/box/enum/abstractNestedClass.kt

@@ -0,0 +1,10 @@
+enum class E {
+    abstract class Nested
+
+    ENTRY
+}
+
+fun box(): String {
+    E.ENTRY
+    return "OK"
+}