Issue 4247 - Cannot create default-constructed struct on heap when constructor is defined

http://d.puremagic.com/issues/show_bug.cgi?id=4247

```d
struct S { int a; }
auto px = new S(2); // fixed bug 6937 (#1353) in 2.061
auto py = new S();  // bug 4247, yet not fixed...
```