Review over a large number of APIs has found that most of the time, the
result type of an Objective-C completion handler method that becomes
"async throws" should be optional. Change the default behavior to
match this, and replace _Nullable_on_error with _Nullable_result to
capture the case where the result should be optional.
When concurrency is enabled, the same Objective-C method on a protocol can
be imported as two different protocol requirements, both of which have the
same selector. When performing conformance checking, only treat a
given @objc protocol requirement as "unsatisfied" by a conformance if
none of the requirements with the same Objective-C selector (+
instance/class designation) are satisfied.
