Concurrency: Fix a condfail in default AsyncIteratorProtocol.next().

Older compilers that do not enable `OptionalIsolatedParameters` by default must
be able to build the `_Concurrency` module.

Resolves rdar://126215750

Author: tshortli

stdlib/public/Concurrency/AsyncIteratorProtocol.swift

@@ -133,6 +133,10 @@ extension AsyncIteratorProtocol {
   @available(SwiftStdlib 6.0, *)
   @inlinable
   public mutating func next() async throws(Failure) -> Element? {
+#if $OptionalIsolatedParameters
     return try await next(isolation: nil)
+#else
+    fatalError("unsupported compiler")
+#endif
   }
 }