[stdlib] Define placeholder version numbers for SwiftStdlib 5.7

Also update existing declarations with SwiftStdlib 9999 availability to use the 5.7 name.

Author: lorentey

stdlib/public/Concurrency/Task.swift

@@ -301,7 +301,7 @@ extension Task where Success == Never, Failure == Never {
   /// The current task's base priority.
   ///
   /// If you access this property outside of any task, this returns nil
-  @available(SwiftStdlib 9999, *)
+  @available(SwiftStdlib 5.7, *)
   public static var basePriority: TaskPriority? {
     withUnsafeCurrentTask { task in
       // If we are running on behalf of a task, use that task's priority.

utils/availability-macros.def

@@ -31,6 +31,7 @@ SwiftStdlib 5.3:macOS 11.0, iOS 14.0, watchOS 7.0, tvOS 14.0
 SwiftStdlib 5.4:macOS 11.3, iOS 14.5, watchOS 7.4, tvOS 14.5
 SwiftStdlib 5.5:macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0
 SwiftStdlib 5.6:macOS 12.3, iOS 15.4, watchOS 8.5, tvOS 15.4
+SwiftStdlib 5.7:macOS 9999, iOS 9999, watchOS 9999, tvOS 9999
 
 # Local Variables:
 # mode: conf-unix