Combining ? with throwing when there's no reasonable default

[Joe_Groff]

@John_McCall

I don't think we'd want to make arbitrary uninhabited types act like bottom in the type system; that would almost certainly do nasty things to the type-checker.

Allowing specifically Never to act like bottom seems tractable, though, and is a direction we suggested going forward in our resolution for rejecting the !! operator. That combined with making throw into an expression sounds good to me.