[libcxx] Fix ctype_byname<wchar_t>::do_is() mask checking.

This patch: http://reviews.llvm.org/D5081
Original patch: http://reviews.llvm.org/D5071 (from @danalbert)


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@216497 91177308-0d34-0410-b5e6-96231b3b80d8