Set `ImpredicativeTypes` during :print command. (#14828)

If ImpredicativeTypes is not enabled, then :print <term> will fail if the type of <term> has nested foralls or =>s. This is because the GHCi debugger's internals will attempt to unify a metavariable with the type of <term> and then display the result, but if the type has nested foralls or =>s, then unification will fail. As a result, :print will bail out and the unhelpful result will be <term> = (_t1::t1) (where t1 is a metavariable).

Beware: <term> can have nested foralls even if its definition doesn't use RankNTypes! Here is an example from #14828 (closed):

  class Functor f where
    fmap :: (a -> b) -> f a -> f b

Somewhat surprisingly, :print fmap considers the type of fmap to have nested foralls. This is because the GHCi debugger sees the type fmap :: forall f. Functor f => forall a b. (a -> b) -> f a -> f b. We could envision deeply instantiating this type to get the type forall f a b. Functor f => (a -> b) -> f a -> f b, but this trick wouldn't work for higher-rank types.

Instead, we adopt a simpler fix: enable ImpredicativeTypes when using :print and friends in the GHCi debugger. This is allows metavariables to unify with types that have nested (or higher-rank) foralls/=>s, which makes :print fmap display as fmap = (_t1::forall a b. Functor f => (a -> b) -> f a -> f b), as expected.

Although ImpredicativeTypes is a somewhat unpredictable from a type inference perspective, there is no danger in using it in the GHCi debugger, since all of the terms that the GHCi debugger deals with have already been typechecked.