[swift-evolution] Make non-void functions @warn_unused_result by default

[Matthew Johnson]

> 
> There are a number of operations that sensibly return values, but are frequently used only for side effects, such as `pop` on a stack. I agree they're in the minority, and @warn_unused_result's polarity is backwards, but they're a big enough minority that the inverse attribute is valuable.

Sure, that makes sense as long as it’s used relatively sparingly! :)