Fix for PR#6441 (Stefan Holdermans)

During closure conversion, the right-hand side of a functional binding in a
recursive binding group is now always closed as a named expression rather than
an anonymous expression.  As a result, direct recursive calls do not need to
retrieve their target from a closure environment.  This does not only result in
more efficient code being generated, but also avoids a potential blocker for
successful tail-call detection during pseudo-instruction selection.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14966 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02