[Concurrency] add TaskLocal::Item:iisParentPointer for cleaner API

Author: ktoso

Diff for: include/swift/ABI/TaskLocal.h

@@ -69,6 +69,15 @@ class TaskLocal {
     // Trailing storage for the value itself. The storage will be
     // uninitialized or contain an instance of \c valueType.
 
+    /// Returns true if this item is a 'parent pointer'.
+    ///
+    /// A parent pointer is special kind of `Item` is created when pointing at
+    /// the parent storage, forming a chain of task local items spanning multiple
+    /// tasks.
+    bool isParentPointer() const {
+      return !valueType;
+    }
+
   protected:
     explicit Item()
       : next(0),

Diff for: stdlib/public/Concurrency/TaskLocal.cpp

@@ -224,7 +224,7 @@ void TaskLocal::Item::copyTo(AsyncTask *target) {
   // 'parent' pointers are signified by null valueType.
   // We must not copy parent pointers, but rather perform a deep copy of all values,
   // as such, we skip parent pointers here entirely.
-  if (!valueType)
+  if (isParentPointer())
     return;
 
   auto item = Item::createLink(target, key, valueType);

Diff for: test/Concurrency/Runtime/async_task_locals_copy_to_async.swift

@@ -139,6 +139,9 @@ func test_unstructured_noValues_childTasks() async {
       try await downloadImage(from: "")
     }
   }
+
+  // these child tasks have a parent pointer in their task local storage.
+  // we must not copy it when performing the copyTo for a new unstructured task.
   async let one = work()
   async let two = work()
   async let three = work()