Change char_traits<char16_t>::eof() to return 0xFFFF instead of 0xDFFF. Fixes PR#24342

llvm-svn: 243937
diff --git a/libcxx/include/string b/libcxx/include/string
index 5777ee2..2c677ce 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -742,7 +742,7 @@
     static inline _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
         {return __c1 == __c2;}
     static inline _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT
-        {return int_type(0xDFFF);}
+        {return int_type(0xFFFF);}
 };
 
 inline _LIBCPP_INLINE_VISIBILITY