Allow an optional 'when' clause after a pattern payload.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13920 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
svn did not add these files during previous merge.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13921 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02