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

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

include/swift/ABI/Executor.h

@@ -208,6 +208,12 @@ class TaskExecutorRef {
 
 public:
 
+  // Only public for CompatibilityOverrideConcurrency stubs.
+  // Prefer `TaskExecutorRef::undefined` instead.
+  explicit TaskExecutorRef() : Identity(nullptr), Implementation(0) {
+    assert(isUndefined());
+  }
+
   constexpr static TaskExecutorRef undefined() {
     return TaskExecutorRef(nullptr, 0);
   }
@@ -232,6 +238,8 @@ public:
     return Identity == 0;
   }
 
+  bool isDefined() const { return !isUndefined(); }
+
   TaskExecutorKind getExecutorKind() const {
     return static_cast<TaskExecutorKind>(Implementation & ~WitnessTableMask);
   }