Generate equals/hashCode(): Enable for classes without properties

Also:
- forbid for enum classes
- use javaClass.hashCode() instead of 0 if class has no properties

 #KT-18418 Fixed

idea/testData/codeInsight/generate/equalsWithHashCode/explicitDefaultAccessors.kt.after

@@ -12,9 +12,7 @@ class Test {
     }
 
     override fun hashCode(): Int {
-        return 0
+        return javaClass.hashCode()
     }
 
-}
-
-// FORCED
+}