These io_error asserts that std::errc::is_a_directory has message "Is a directory". On MSVC++ it reports "is a directory" (with a lowercase I). That doesn't matter for the ios_failure component being tested, so just implement in terms of system_category().message().
Reviewed as https://reviews.llvm.org/D45715
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@330791 91177308-0d34-0410-b5e6-96231b3b80d8
