Opaque aggregate declarations will shadow later detailled declarations.

The following fails:

`a.d`:
```d
module a;
struct A;
struct A
{
    int member;
}
```

`b.d`:
```d
import a;typeof(A.
```

```sh
dcd-server -I. &
dcd-client b.d -c18 | grep member
```

But works fine if the first opaque struct declaration is removed.

I believe this is related to #397.