Fix a race condition which leads product to hang

the order to read this.joinableTask is not ensured in the current lock-free part of the logic.
Under a race condition, this leads AsyncLazy to wait the inner task instead of the joinable task when two threads hitting this code about the same time.

Author: Lifeng Lu

src/Microsoft.VisualStudio.Threading/AsyncLazy`1.cs

@@ -224,6 +224,10 @@ public Task<T> GetValueAsync(CancellationToken cancellationToken)
                             // to synchronously block the Main thread waiting for the result
                             // without leading to deadlocks.
                             this.joinableTask = this.jobFactory.RunAsync(valueFactory);
+
+                            // this ensures that this.joinableTask must be committed before this.value
+                            Thread.MemoryBarrier();
+
                             this.value = this.joinableTask.Task;
                         }
                         else
@@ -245,6 +249,9 @@ public Task<T> GetValueAsync(CancellationToken cancellationToken)
             resumableAwaiter?.Resume();
         }
 
+        // this ensures that this.joinableTask cannot be retrieved before the conditional check using this.value
+        Thread.MemoryBarrier();
+
         return this.joinableTask?.JoinAsync(continueOnCapturedContext: false, cancellationToken) ?? this.value.WithCancellation(cancellationToken);
     }