Use a version of the coverage condition even with UndecidableInstances.

This fixes bug #1241 and #2247.  When UndecidableInstances are on,
we use the "Liberal Coverage Condition", which is what GHC used to do in
the past.  This is the gist of the check:

class C a b | a -> b
instance theta => C t1 t2

we check that `fvs t2` is a subset of `fd-closure(theta,fvs t1)`.

This is strictly more general than the coverage condition, while
it still guarantees consistency with the FDs of the class.  This
check is completely orthogonal to termination (it by no means guarantees
it).

I am not sure of the role of the "coverage condition" in termination---
the comments suggest that it is important.  This is why, for the moment,
we only use this check when UndecidableInstances are on.