Bitfields: .init value contains void elements

i.e:
```d
struct T
{
    int a = 4;
    int b : 16;
    int c : 16;
    int d = 8;
}
pragma(msg, T.init);
```
Prints:
```
T(4, 0, , 8)
```

Likewise
```d
static assert(T.init == T(4,0,0,8));
```
Errors with
```
Error: integer constant expression expected instead of `void`
    int c : 16;
            ^
```