xAODCore: Work around cling problems.
In some cases, some dictionary load requests will fail if they are done in the `wrong' order. This is a cling bug (as of root 6.20.00), but we can work around it here. We add a TClassGenerator so that if, for example, we're trying to load ElementLink, we first load T. And similarly for a few other Athena template classes. This is observed to be sufficient to fix the order-dependent failures. See ATR-22681.
Also instantiate the BaseInfo specializations for containers in dictionaries. This will be needed to work around other cling bugs which are causing assertion failures in the dbg build.