Update task cancellation API based on review feedback

Swap the order of the arguments so we now have:

    await withTaskCancellationHandler {
      // do stuff
    } onCancel: {
      // ..
    }

Author: DougGregor

stdlib/public/Concurrency/SourceCompatibilityShims.swift

@@ -46,6 +46,15 @@ extension TaskPriority {
   }
 }
 
+@available(SwiftStdlib 5.5, *)
+@_alwaysEmitIntoClient
+public func withTaskCancellationHandler<T>(
+  handler: @Sendable () -> (),
+  operation: () async throws -> T
+) async rethrows -> T {
+  try await withTaskCancellationHandler(operation: operation, onCancel: handler)
+}
+
 @available(SwiftStdlib 5.5, *)
 extension Task where Success == Never, Failure == Never {
   @available(*, deprecated, message: "`Task.withCancellationHandler` has been replaced by `withTaskCancellationHandler` and will be removed shortly.")

stdlib/public/Concurrency/TaskCancellation.swift

@@ -29,8 +29,8 @@ import Swift
 /// This function returns instantly and will never suspend.
 @available(SwiftStdlib 5.5, *)
 public func withTaskCancellationHandler<T>(
-  handler: @Sendable () -> (),
-  operation: () async throws -> T
+  operation: () async throws -> T,
+  onCancel handler: @Sendable () -> Void
 ) async rethrows -> T {
   let task = Builtin.getCurrentAsyncTask()
 

test/Concurrency/async_cancellation.swift

@@ -30,9 +30,10 @@ func test_cancellation_withTaskCancellationHandler(_ anything: Any) async -> Pic
   let handle: Task<PictureData, Error> = .init {
     let file = SomeFile()
 
-    return await withTaskCancellationHandler(
-      handler: { file.close() }) {
+    return await withTaskCancellationHandler {
       await test_cancellation_guard_isCancelled(file)
+    } onCancel: {
+      file.close()
     }
   }