swift

9e875f98 - Concurrency: Declare explicit Failure typealiases on AsyncSequence iterators.

Concurrency: Declare explicit Failure typealiases on AsyncSequence iterators. Associated type inference ought to take care of providing the `Failure` typealias for these `AsyncIteratorProtocol` types. However, the inferred typealias is printed with `@_implements` in the `.swiftinterface`, guarded with the `$AssociatedTypeImplements` language feature guard, which means older compilers cannot see the typealias and therefore think the conformance is incomplete. To make sure the `_Concurrency` module's interface is backward compatible, we must manually define these typealiases temporarily. Part of rdar://125138945