Support binding NSObjects as IntPtr. This is usually not
a problem, because when we fetch the ObjC signature for a
method, we usually get the signature as exported by us,
(in which case a parameter bound as 'IntPtr' would be treated
as 'void *' in the dynamic registrar) *except* when the
selector corresponds with a protocol the type implements,
in which case we get the signature as defined in the protocol.
https://bugzilla.xamarin.com/show_bug.cgi?id=41132