wchar_t is not as portable as char32_t. On Windows, wchar_t is
16-bytes and on Linux 32-bits. The conversion to utf8 causes the
characters to exceed the limits on char16_t, resulting in tautological
comparisons.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294917 91177308-0d34-0410-b5e6-96231b3b80d8