In `Convert`, we were incorrectly using `NotPromoted` to
denote type constructors that were actually intended to be promoted,
resulting in poor `-ddump-splices` output (as seen in #15243).
Easily fixed.

Test Plan: make test TEST=T15243

Reviewers: bgamari, goldfire

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15243

Differential Revision: https://phabricator.haskell.org/D4809