[Concurrency] Freeze JobPriority, comment that Flags location is assumed (#64267)

Author: ktoso

include/swift/ABI/Task.h

@@ -72,9 +72,12 @@ class alignas(2 * alignof(void*)) Job :
   // Reserved for the use of the scheduler.
   void *SchedulerPrivate[2];
 
+  /// WARNING: DO NOT MOVE.
+  /// Schedulers may assume the memory location of the Flags in order to avoid a runtime call
+  /// to get the priority of a job.
   JobFlags Flags;
 
-  // Derived classes can use this to store a Job Id.
+  /// Derived classes can use this to store a Job Id.
   uint32_t Id = 0;
 
   /// The voucher associated with the job. Note: this is currently unused on

stdlib/public/Concurrency/PartialAsyncTask.swift

@@ -63,6 +63,7 @@ public struct UnownedJob: Sendable {
 ///
 /// Conversions between the two priorities are available as initializers on the respective types.
 @available(SwiftStdlib 5.9, *)
+@frozen
 public struct JobPriority {
   public typealias RawValue = UInt8