Fix nested references to 'ref' foreach variables. These "walk around" the array being iterated over, so they're a bit trickier than other variables to get right.

module union1;

union U
{
    float f;
    int i;
}

void main()
{
    float f = 2;
    U u = U(f);
    assert(u.i == *cast(int*)&f);
}