The change in #59479 inadvertently fixed another bug in selector conflict checking: Swift did not notice when an ObjC header declared an async-imported method, but a Swift extension to the same class added another method with the same selector. Unfortunately, fixing this bug was source-breaking, and it also caused Swift to diagnose spurious conflicts between the various names that a single ObjC method might be imported with. Soften the error to a warning in Swift 5 mode and suppress the spurious diagnostics.
Fixes rdar://95887113.
Previously, the method table would contain duplicate copies due to the
ProtocolDecl carrying a completionHandler-based version of the method,
as well as the async version of the method.
Fixes rdar://76192003.
