[libcxx] [test] Remove nonportable that errc::is_a_directory produces "Is a...

[libcxx] [test] Remove nonportable that errc::is_a_directory produces "Is a directory" from ios_base::failure tests

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

llvm-svn: 330791