Prohibit non-inner classes in enum entries since 1.3

compiler/testData/diagnostics/tests/enum/dontCreatePackageTypeForEnumEntry.kt

@@ -1,7 +1,7 @@
 enum class E {
     FIRST,
     SECOND {
-        class A
+        <!NESTED_CLASS_NOT_ALLOWED_SINCE_1_3!>class A<!>
     };
 }
 

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
         }
     }