Don't discard the type definedness when handling parameters.

[LemonBoy]

RT#129005

I'm not 100% happy with the current solution as it allows the user to specify an unsatisfiable superposition of definedness such as in this short example:

constant IntD := Int:D; (sub (IntD:U $a) { ... })(Int)
constant IntD := Int:D; (sub (IntD:U $a) { ... })(42)

Probably it's just fine as-is, it's very much DWIM heh.
Another option would be to throw an exception if the definedness constraints are unsatisfiable.