Clean up some suspicious code. There's no guarantee a constant of integer type is a ConstantInt, so it's not safe to just cast<> these to ConstantInt. (It could be a constant expression, for example) The code is shorter this way too :).

module bug19;

void main()
{
    auto dg = (int i) { return i*2; };
    assert(dg(2) == 4);
}