Recognize both prefixed and unprefixed built-in attributes (e.g. ocaml.deprecated or deprecated).

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14750 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
When the payload of the ocaml.deprecated attribute is a string, report it as part of the warning message.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14751 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02