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

libraries/base/GHC/Show.hs

@@ -215,7 +215,7 @@ instance Show TyCon where
 
 -- | @since 4.9.0.0
 instance Show TrName where
-  showsPrec _ (TrNameS s) = showString (unpackCString# s)
+  showsPrec _ (TrNameS s) = showString (unpackCStringUtf8# s)
   showsPrec _ (TrNameD s) = showString s
 
 -- | @since 4.9.0.0