Cherry pick 15174 from 4.02 (#6529: the checked flag is now part of pers_struct).

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15175 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Merge remote-tracking branch 'ocaml/4.02' into 4.02-gdb