Fix #13948 by being pickier about when to suggest DataKinds

Commit 343cb32d (#13568) made GHC a bit
too cavalier in suggesting when data constructors are in scope (and
suggesting the use of `DataKinds`). This tones down the suggestions so
that `DataKinds` is only suggested if a data constructor of that name is
actually in scope (previously, it would always suggest, even if it was
out of scope).

Fixes #13948.

Test Plan: ./validate

Reviewers: mpickering, austin, bgamari

Reviewed By: mpickering

Subscribers: rwbarton, thomie

GHC Trac Issues: #13948

Differential Revision: https://phabricator.haskell.org/D3719