[swift-evolution] Make distinction between ?? -> T and ?? -> T?

[Joe Groff]

> On Mar 7, 2016, at 2:26 PM, Radosław Pietruszewski via swift-evolution <swift-evolution at swift.org> wrote:
> 
> -1, never seen a real world situation where this would be confusing/problematic.
> 
> Having one operator is nice for chaining:
> 
> var value = someArray ?? someFallback ?? secondaryFallback ?? []

This case could be handled by making `??` right-associative. In fact, I had originally implemented it that way. I don't recall why we changed it and added the extra overload; Jordan, do you recall?

-Joe