The `align` attribute now allows specifying `default` explicitly

A lone `align` sets the alignment to the type’s default.
To be more explicit, `align(default)` does the same.

```
struct S
{
    align(4)
    {
        byte x;
        align(default) long y;
        long z;
    }
}

void main()
{
    pragma(msg, S.x.alignof); // 4
    pragma(msg, S.y.alignof); // 8
    pragma(msg, S.z.alignof); // 4
}
```
