Summary:
This patch fixes #15806, where we found that the `:k` command in GHCi
misses a validity checking for the type.

Missing validity checking causes `:k` to accept types that are not validated.
For example, `:k (Maybe (forall a. a -> a))` (incorrectly) returns `*`, while
impredictivity of type instantiation shouldn't be allowed.

Test Plan: ./validate

Reviewers: simonpj, goldfire, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #15806

Differential Revision: https://phabricator.haskell.org/D5265