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 complex3;

void main()
{
    cfloat c1 = 1f + 0i;
    cfloat c2 = 0f + 0i;
    c2 += 1f + 0i;
    assert(c1 == c2);
}