Fix signatures of fallback tow(upper|lower)_l.

Summary:
These functions take and return wint_t, not int:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/towupper.html

Reviewers: mclow.lists, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D24743

llvm-svn: 281936