Reprise: new lint: Unintentional return of unit from closures expecting Ord

This lint catches cases where the last statement of a closure expecting
an instance of Ord has a trailing semi-colon. It compiles since the
closure ends up return () which also implements Ord but causes
unexpected results in cases such as sort_by_key.

Fixes rust-lang#5080

Reprise of rust-lang#5348 where I addressed all the comments there

changelog: add lint [`unit_return_expecting_ord`]