[DDMD] Print all braces in impcnv tables

Print braces in the generated tables.

The C array literal style:

```
struct S { int a, b, c; }
S s[2] =
{
   1, 2, 3,
   4, 5, 6
};
```
Is annoying to convert to the D version:
```
S[2] s =
[
   S(1, 2, 3),
   S(4, 5, 6)
];
```
Because then the tool need to know the number of members in the struct.  It's trivial to just add the brackets.