Always emit AsyncIteratorProtocol._nextElement into the client

... this allows us to use the entrypoint when back-deploying code that
uses the async for..in loop.

Author: DougGregor

stdlib/public/Concurrency/AsyncIteratorProtocol.swift

@@ -112,7 +112,7 @@ public protocol AsyncIteratorProtocol<Element, Failure> {
 extension AsyncIteratorProtocol {
   /// Default implementation of `_nextElement()` in terms of `next()`, which is
   /// required to maintain backward compatibility with existing async iterators.
-  @available(SwiftStdlib 5.11, *)
+  @_alwaysEmitIntoClient
   public mutating func _nextElement() async throws(Failure) -> Element? {
     do {
       return try await next()

test/api-digester/stability-concurrency-abi.test

@@ -65,6 +65,7 @@ Protocol AsyncSequence has generic signature change from <Self.AsyncIterator : _
 Struct CheckedContinuation has removed conformance to UnsafeSendable
 Protocol AsyncIteratorProtocol is now without @rethrows
 Protocol AsyncSequence is now without @rethrows
+Func AsyncIteratorProtocol._nextElement() has been added as a protocol requirement
 
 // SerialExecutor gained `enqueue(_: __owned Job)`, protocol requirements got default implementations
 Func SerialExecutor.enqueue(_:) has been added as a protocol requirement