[Concurrency] Update Task and Group APIs based on Review 1

Author: ktoso

include/swift/ABI/Task.h

@@ -566,7 +566,7 @@ class ContinuationAsyncContext : public AsyncContext {
 /// task.
 ///
 /// This type matches the ABI of a function `<T> () async throws -> T`, which
-/// is the type used by `Task.runDetached` and `Task.group.add` to create
+/// is the type used by `spawnDetached` and `Task.group.add` to create
 /// futures.
 class FutureAsyncContext : public AsyncContext {
 public:

stdlib/public/Concurrency/Task.cpp

@@ -102,7 +102,6 @@ void AsyncTask::completeFuture(AsyncContext *context) {
       reinterpret_cast<char *>(context) - sizeof(FutureAsyncContextPrefix));
   bool hadErrorResult = false;
   auto errorObject = asyncContextPrefix->errorResult;
-//   printf("asyncTask::completeFuture errorObject: %p\n", errorObject);
   fragment->getError() = errorObject;
   if (errorObject) {
     hadErrorResult = true;

stdlib/public/Concurrency/Task.swift

@@ -25,7 +25,9 @@ extension Task {
   ///
   /// - SeeAlso: `checkCancellation()`
   public static var isCancelled: Bool {
-     Task.unsafeCurrent?.isCancelled ?? false
+     withUnsafeCurrentTask { task in
+       task?.isCancelled ?? false
+     }
   }
 
   /// Returns `true` if the task is cancelled, and should stop executing.