Previously, we were skipping over `$tcUnit#` entirely when
wiring in `Typeable` tycons, resulting in #14811. Easily fixed.

Test Plan: make test TEST=T14811

Reviewers: bgamari, dfeuer

Reviewed By: dfeuer

Subscribers: dfeuer, rwbarton, thomie, carter

GHC Trac Issues: #14811

Differential Revision: https://phabricator.haskell.org/D4414

(cherry picked from commit d5ac5820)