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.

unittests/runtime/CompatibilityOverrideConcurrency.cpp

@@ -179,10 +179,6 @@ TEST_F(CompatibilityOverrideConcurrencyTest, test_swift_taskGroup_initialize) {
   swift_taskGroup_initialize(nullptr);
 }
 
-TEST_F(CompatibilityOverrideConcurrencyTest, test_swift_taskGroup_create) {
-  swift_taskGroup_create();
-}
-
 TEST_F(CompatibilityOverrideConcurrencyTest, test_swift_taskGroup_attachChild) {
   swift_taskGroup_attachChild(nullptr, nullptr);
 }