xAODTracking: Don't hide data member from cling.

Don't hide the data member in TrackParticle_v1 from cling. If we do, then objects created from python can have a layout inconsistent with C++, leading to memory corruption when they are deleted. However, if Cling sees the actual CachedValue type, we get mysterious cling parsing failures. So instead have Cling see just padding of the proper size. (This works becasue the member is transient and all methods referencing it are out-of-line).

See !63818 (merged).