Hide messy stuff from rootcling

A recently introduced bug in ROOT6 causes non-fatal runtime error messages because there are missing forward declarations corresponding to the dictionary in FWCore/MessageLogger.
This pull request hides the offending types from rootcling (genreflex), so the error messages are not observed.
This workaround can remain permanently, because the types in question are not persisted, and do not need to be known to rootcling.
Please merge this pull request as soon as convenient.