Jacques Garrigue pointed out that GHC 7.6 was accepting GADT programs
that our OutsideIn paper said would be rejected.  The reason was that
approximateWC was being too aggressive about floating; see Note
[approximateWC] in TcSimplify.

This simple patch fixes that, but it is (of course) not backward
compatible; a few GADT programs that were (erroneously) accepted
before are now rejected, and will need a type signature.  This could
be under flag control, but I'd rather just make it compulsory.

Example
   data T a where
     TInt :: T Int
     MkT :: T a

   f TInt = 3::Int

Here f (with no type signature) is current accepted, with inferred type
 f :: T a -> Int
but should be rejected