Factorize code to print constructors.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15508 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Sync with trunk.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/constructors_with_record4@15509 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02