Combining ? with throwing when there's no reasonable default

[Chris_Lattner3 (Chris Lattner)]

@John_McCall

I did actually originally have throw as an bottom expression, but I got grief over it and didn't care to push the issue. It seems like a reasonable thing to do.

I agree. Several other things were different back in Swift 1 days while we were discussing this - e.g. we didn't have Never at the time. I'm +1 on moving throw to being an expression.