base: Fix Unicode handling of TyCon's Show instance

Test Plan: `make test TEST=T14925`

Reviewers: hvr, dfeuer

Reviewed By: dfeuer

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14925

Differential Revision: https://phabricator.haskell.org/D4530

(cherry picked from commit 20ae19fc)