[swift-evolution] Warning on the '(T...) -> U' to '(T...) -> ()' conversion

[Joe Groff]

Javier Soto brought up a case where his ReactiveCocoa code silently did the wrong thing by implicitly discarding the result of a closure:

https://twitter.com/Javi/status/695680700033306624

Since it sounds like we plan on making `@warn_unused_result` the default, it seems natural to also warn when a closure has its result type converted away to Void. If there's one 'return' statement, we could dive into the return expression to decide whether to warn based on the function whose result is getting ignored.

-Joe