Prohibit non-inner classes in enum entries since 1.3

compiler/testData/diagnostics/tests/enum/inner/insideEnumEntry.kt

@@ -1,6 +1,6 @@
 enum class E {
     ABC {
-        enum class F {
+        <!NESTED_CLASS_NOT_ALLOWED_SINCE_1_3!>enum class F<!> {
             DEF
         }
     }