[Concurrency] Make Job/AsyncTask minimally compatible with dispatch object layout

Create a TargetDispatchClassMetadata for Swift metadata that also has a dispatch-compatible vtable. Dispatch leaves room for ObjC class metadata so the two regions don't overlap. (The vtable currently consists of a single dummy entry; this will be filled out later.)

Rearrange the Job and AsyncTask hierarchy so that AsyncTask inherits only from Job, which in turn inherits from HeapObject. This gives all Job instances a dispatch-compatible isa field. It also gives them a refcount word, which is wasted on instances that aren't AsyncTask instances. Maybe we can find some use for that space in the future.

rdar://75227953

lib/IRGen/IRGenModule.cpp

@@ -619,8 +619,10 @@ IRGenModule::IRGenModule(IRGenerator &irgen,
   SwiftTaskGroupPtrTy = Int8PtrTy; // we pass it opaquely (TaskGroup*)
   SwiftExecutorPtrTy = SwiftExecutorTy->getPointerTo(DefaultAS);
   SwiftJobTy = createStructType(*this, "swift.job", {
+    RefCountedStructTy,   // object header
+    Int8PtrTy, Int8PtrTy, // SchedulerPrivate
     SizeTy,               // flags
-    Int8PtrTy             // execution function pointer
+    FunctionPtrTy,        // RunJob/ResumeTask
   });
   SwiftJobPtrTy = SwiftJobTy->getPointerTo();