(this is number 2 of 9 commits to be applied together)

  Type constructors now carry information on the variance (positive
  and/or negative) of each of their type arguments (tyConArgVrcs).
  This information is provided for primitive types and computed for
  others.  If a tycon has been imported abstractly and this variance
  information is subsequently demanded, we make a pessimistic
  assumption and warn that -fno-prune-tydecls should be used.