We now use this function rather than Outputable.{printSDoc,printErrs}.

Outputable is arguably a better home for the function, but putting it
in DynFlags should dissuade people from using it inappropriately (in
particular, nothing other than the default hooks should have stdout
or stderr hardcoded).

Not exporting it at all would also be an option, but exporting it with
an ungainly name will make it slightly easier for people who want to
send output to other Handles for some reason.