Address a few more warnings in the concurrency library

Author: DougGregor

stdlib/public/Concurrency/AsyncStream.swift

@@ -175,15 +175,15 @@ public struct AsyncStream<Element> {
     let storage: _AsyncStreamCriticalStorage<Optional<() async -> Element?>>
       = .create(produce)
     self.produce = {
-      return await Task.withCancellationHandler {
-        storage.value = nil
-        onCancel?()
-      } operation: {
+      return await withTaskCancellationHandler {
         guard let result = await storage.value?() else {
           storage.value = nil
           return nil
         }
         return result
+      } onCancel: {
+        storage.value = nil
+        onCancel?()
       }
     }
   }

stdlib/public/Concurrency/TaskCancellation.swift

@@ -32,8 +32,6 @@ public func withTaskCancellationHandler<T>(
   operation: () async throws -> T,
   onCancel handler: @Sendable () -> Void
 ) async rethrows -> T {
-  let task = Builtin.getCurrentAsyncTask()
-
   // unconditionally add the cancellation record to the task.
   // if the task was already cancelled, it will be executed right away.
   let record = _taskAddCancellationHandler(handler: handler)

stdlib/public/Concurrency/TaskLocal.swift

@@ -188,6 +188,7 @@ public final class TaskLocal<Value: Sendable>: Sendable, CustomStringConvertible
   // the type-checker that this property-wrapper never wants to have an enclosing
   // instance (it is impossible to declare a property wrapper inside the `Never`
   // type).
+  @available(*, unavailable, message: "property wrappers cannot be instance members")
   public static subscript(
     _enclosingInstance object: Never,
     wrapped wrappedKeyPath: ReferenceWritableKeyPath<Never, Value>,