[5.9] Handle async vs. completion-handler mismatches in @_objcImplementation

[DougGregor]

• Explanation: When matching methods in an @_objcImplementation extension against an Objective-C method imported as async, we were not accounting for the fact that such methods can be imported both as both async and with a completion handler, and therefore complained that one of the two Objective-C method signatures was not satisfied. Allow both of those requirements to be satisfied by a single candidate, whether it's of the async or completion-handler form.
• Scope: Affects code using the experimental @_objcImplementation feature with Objective-C methods that are imported as async.
• Risk: Low, due to narrow scope and this loosening checking for certain methods.
• Issue: rdar://108160837
• Testing: Additional tests added.
• Original pull request: Handle async vs. completion-handler mismatches in @_objcImplementation. #65253

[DougGregor]

@swift-ci please test