[K/N][IR] Never consider a enum a final class

Unfortunately, neither descriptors nor Lazy IR have enough information
to correctly deduce a enum's modality as psi2ir does, so to avoid the RTTI generator
generating different vtables in different setups (whether a klib is cached or not),
conservatively consider all enums non-final.

kotlin-native/backend.native/tests/codegen/enum/openMethodNoOverrides_lib.kt

@@ -0,0 +1,10 @@
+/*
+ * Copyright 2010-2022 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the LICENSE file.
+ */
+
+enum class Foo {
+    Z;
+
+    open fun bar() = 42
+}

kotlin-native/backend.native/tests/codegen/enum/openMethodNoOverrides_main.kt

@@ -0,0 +1,6 @@
+/*
+ * Copyright 2010-2022 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the LICENSE file.
+ */
+
+fun main() = println(Foo.Z.bar())

kotlin-native/backend.native/tests/codegen/enum/openMethodNoOverrides_main.out

@@ -0,0 +1 @@
+42