This avoids the compiler from crashing when, for example, a warning
contains a non-Latin identifier and the LANG variable is set to C.
Fixes #6037.

Test Plan:
Create a Haskell source file containing an identifier with non-Latin
characters and no type signature. Compile with `LANG=C ghc -Wall
foo.hs`, and it should fail. With this patch, it will succeed.

Reviewers: austin, rwbarton, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1153

GHC Trac Issues: #6037, #10762