Clean up the TaskGroup ABI:

- stop storing the parent task in the TaskGroup at the .swift level
- make sure that swift_taskGroup_isCancelled is implied by the parent
  task being cancelled
- make the TaskGroup structs frozen
- make the withTaskGroup functions inlinable
- remove swift_taskGroup_create
- teach IRGen to allocate memory for the task group
- don't deallocate the task group in swift_taskGroup_destroy

To achieve the allocation change, introduce paired create/destroy builtins.

Furthermore, remove the _swiftRetain and _swiftRelease functions and
several calls to them.  Replace them with uses of the appropriate builtins.
I should probably change the builtins to return retained, since they're
working with a managed type, but I'll do that in a separate commit.

lib/IRGen/GenConcurrency.cpp

@@ -170,3 +170,27 @@ void irgen::emitGetCurrentExecutor(IRGenFunction &IGF, Explosion &out) {
 
   IGF.emitAllExtractValues(call, IGF.IGM.SwiftExecutorTy, out);
 }
+
+llvm::Value *irgen::emitCreateTaskGroup(IRGenFunction &IGF) {
+  auto ty = llvm::ArrayType::get(IGF.IGM.Int8PtrTy, NumWords_TaskGroup);
+  auto address = IGF.createAlloca(ty, Alignment(Alignment_TaskGroup));
+  auto group = IGF.Builder.CreateBitCast(address.getAddress(),
+                                         IGF.IGM.Int8PtrTy);
+  IGF.Builder.CreateLifetimeStart(group);
+
+  auto *call = IGF.Builder.CreateCall(IGF.IGM.getTaskGroupInitializeFn(),
+                                      {group});
+  call->setDoesNotThrow();
+  call->setCallingConv(IGF.IGM.SwiftCC);
+
+  return group;
+}
+
+void irgen::emitDestroyTaskGroup(IRGenFunction &IGF, llvm::Value *group) {
+  auto *call = IGF.Builder.CreateCall(IGF.IGM.getTaskGroupDestroyFn(),
+                                      {group});
+  call->setDoesNotThrow();
+  call->setCallingConv(IGF.IGM.SwiftCC);
+
+  IGF.Builder.CreateLifetimeEnd(group);
+}