This is bug or not? (immutable class containing struct with dtor)

[Denis Feklushkin]

```d
/+ dub.json:
{
   "name": "test",
   "dependencies": {
   }
}
+/

struct S
{
     ~this() {}
}

immutable class Imm
{
     S s; // this is immutable value because whole class is 
immutable

     this()
     {
         s = S();
     }

     ~this() {} // Comment out this to fix this compilation  error:
     // Error: `immutable` method `serializer_bug.Imm.~this` is 
not callable using a mutable object
     // What mutable object is meant here?
}


void main()
{
     auto ic = new immutable Imm();
}
```

Run: $ dub --single bug.d