operator<<(ostream,StringData) ignores stream state.

ostream << std::setw(5) << expr

behaves differently depending on whether expr is a std::string or a StringData.

libc++'s operator<<(ostream, string_view) is complex, but accessible for reuse.
https://github.com/llvm-mirror/libcxx/blob/0f25cd9/include/ostream#L714