Fixes #19692.

Prototypical cases:

    class C1 a where
      x1 :: a -> Int
    data G1 = G1 deriving C1

    class C2 a where
      x2 :: a -> Int
      x2 _ = 0
    data G2 = G2 deriving C2

Both of these used to give this suggestion, but for C1 the suggestion
would have failed (generated code with undefined methods, which compiles
but warns). Now C2 still gives the suggestion but C1 doesn't.