Require a bang pattern when unlifted types are where/let bound; #3182

For now we only get a warning, rather than an error, because the alex
and happy templates don't follow the new rules yet.

HpcParser.y

 { 
-{-# OPTIONS -w #-}
+{-# OPTIONS -Wwarn #-}
 -- The above warning supression flag is a temporary kludge.
 -- While working on this module you are encouraged to remove it and fix
 -- any warnings in the module. See