fix #44328: method validation for opaque closures (#44335)
I believe it's intentional that for these methods, the `sig` field is
just ignored and always set to `Tuple`. Also fixes a lowering bug I
discovered that would cause errors if `Union` was shadowed.
I have verified that this fixes the reported warnings.
Co-authored-by: Jameson Nash <vtjnash@gmail.com>
