Fix Trac #1037

Fix Trac #1037

It makes *sense* for a foreign import to have a zero-sized return, thus

  foreign import ccall foo :: Int -> State# RealWorld

but it's not clear that it's useful, and it requires some back-end (a
Hint for void types) to make it go right through. It's not clear that
we really want this facility, so rather than fixing the code
generator, I'm just making the construct illegal for now.