Fix bit manipulation in ActiveTaskStatus::withEscalatedPriority().

Due to a missing `~` when trying to mask in a new priority + the
`IsEscalated` flag, we were instead getting an incorrect priority as
well as dropping other useful bits. This led to assertions about the
running state of a task not being set.

Author: DougGregor

stdlib/public/Concurrency/TaskPrivate.h

@@ -236,7 +236,7 @@ class alignas(sizeof(void*) * 2) ActiveTaskStatus {
   ActiveTaskStatus withEscalatedPriority(JobPriority priority) const {
     assert(priority > getStoredPriority());
     return ActiveTaskStatus(Record,
-                            (Flags & PriorityMask)
+                            (Flags & ~PriorityMask)
                                | IsEscalated | uintptr_t(priority));
   }
   ActiveTaskStatus withoutStoredPriorityEscalation() const {