Initial Task Executor implementation Task(on:), addTask(on:) etc. (#68793)

Co-authored-by: John McCall <rjmccall@gmail.com>

include/swift/ABI/MetadataValues.h

@@ -2437,6 +2437,7 @@ public:
 /// Flags for schedulable jobs.
 class JobFlags : public FlagSet<uint32_t> {
 public:
+  // clang-format off
   enum {
     Kind           = 0,
     Kind_width     = 8,
@@ -2448,12 +2449,14 @@ public:
 
     // Kind-specific flags.
 
-    Task_IsChildTask           = 24,
-    Task_IsFuture              = 25,
-    Task_IsGroupChildTask      = 26,
+    Task_IsChildTask                      = 24,
+    Task_IsFuture                         = 25,
+    Task_IsGroupChildTask                 = 26,
     // 27 is currently unused
-    Task_IsAsyncLetTask        = 28,
+    Task_IsAsyncLetTask                   = 28,
+    Task_HasInitialTaskExecutorPreference = 29,
   };
+  // clang-format on
 
   explicit JobFlags(uint32_t bits) : FlagSet(bits) {}
   JobFlags(JobKind kind) { setKind(kind); }
@@ -2485,6 +2488,9 @@ public:
   FLAGSET_DEFINE_FLAG_ACCESSORS(Task_IsAsyncLetTask,
                                 task_isAsyncLetTask,
                                 task_setIsAsyncLetTask)
+  FLAGSET_DEFINE_FLAG_ACCESSORS(Task_HasInitialTaskExecutorPreference,
+                                task_hasInitialTaskExecutorPreference,
+                                task_setHasInitialTaskExecutorPreference)
 };
 
 /// Kinds of task status record.
@@ -2510,6 +2516,10 @@ enum class TaskStatusRecordKind : uint8_t {
   /// escalated.
   EscalationNotification = 4,
 
+  /// A task executor preference, which may impact what executor a task will be
+  /// enqueued on.
+  TaskExecutorPreference = 5,
+
   // Kinds >= 192 are private to the implementation.
   First_Reserved = 192,
   Private_RecordLock = 192
@@ -2518,12 +2528,12 @@ enum class TaskStatusRecordKind : uint8_t {
 /// Kinds of option records that can be passed to creating asynchronous tasks.
 enum class TaskOptionRecordKind : uint8_t {
   /// Request a task to be kicked off, or resumed, on a specific executor.
-  Executor  = 0,
+  InitialTaskExecutor = 0,
   /// Request a child task to be part of a specific task group.
   TaskGroup = 1,
   /// DEPRECATED. AsyncLetWithBuffer is used instead.
   /// Request a child task for an 'async let'.
-  AsyncLet  = 2,
+  AsyncLet = 2,
   /// Request a child task for an 'async let'.
   AsyncLetWithBuffer = 3,
   /// Information about the result type of the task, used in embedded Swift.