Fix panic on :kind _ in GHCi (Trac #9879)

Summary:
Running `:kind _` in GHCi produced a panic, fix it by extracting the
wildcards. Now, `:kind _` produces `_ :: k0`. Unfortunately, a `0` is
added after the kind is tidied and I haven't found a way to get rid of
it...

This does not fix the other panic involving TemplateHaskell mentioned
in #9879.

Test Plan: new test GHCiWildcardKind should pass

Reviewers: austin, simonpj

Reviewed By: austin

Subscribers: simonpj, carter, thomie, monoidal

Differential Revision: https://phabricator.haskell.org/D572

GHC Trac Issues: #9879

(cherry picked from commit 6eb86a56)