Move CancellationError out to the top level

(cherry picked from commit fdee0ff)

Author: DougGregor

stdlib/public/Concurrency/SourceCompatibilityShims.swift

@@ -18,11 +18,17 @@ import Swift
 
 @available(SwiftStdlib 5.5, *)
 extension Task where Success == Never, Failure == Never {
-  @available(*, deprecated, message: "use TaskPriority")
+  @available(*, deprecated, message: "Task.Priority has been removed; use TaskPriority")
   public typealias Priority = TaskPriority
 
-  @available(*, deprecated, message: "use Task")
+  @available(*, deprecated, message: "Task.Handle has been removed; use Task")
   public typealias Handle = _Concurrency.Task
+
+  @available(*, deprecated, message: "Task.CancellationError has been removed; use CancellationError")
+  @_alwaysEmitIntoClient
+  public static func CancellationError() -> _Concurrency.CancellationError {
+    return _Concurrency.CancellationError()
+  }
 }
 
 @available(SwiftStdlib 5.5, *)

stdlib/public/Concurrency/TaskCancellation.swift

@@ -89,17 +89,16 @@ extension Task where Success == Never, Failure == Never {
       throw CancellationError()
     }
   }
+}
 
-  /// The default cancellation thrown when a task is cancelled.
-  ///
-  /// The `Task.checkCancellation()` method throws this error
-  /// if the current task has been canceled.
-  /// You can throw this error in your cancellation-checking code,
-  /// or another more specific error if needed.
-  public struct CancellationError: Error {
-    // no extra information, cancellation is intended to be light-weight
-    public init() {}
-  }
+/// The default cancellation thrown when a task is cancelled.
+///
+/// This error is also thrown automatically by `Task.checkCancellation()`,
+/// if the current task has been cancelled.
+@available(SwiftStdlib 5.5, *)
+public struct CancellationError: Error {
+  // no extra information, cancellation is intended to be light-weight
+  public init() {}
 }
 
 @available(SwiftStdlib 5.5, *)

test/Concurrency/async_task_groups.swift

@@ -16,7 +16,7 @@ func asyncThrowsOnCancel() async throws -> Int {
     await Task.sleep(1_000_000_000)
   }
 
-  throw Task.CancellationError()
+  throw CancellationError()
 }
 
 @available(SwiftStdlib 5.5, *)