[libc++] Lift std::errc into a separated header
Summary: This is needed to implement <charconv>, otherwise <charconv> would need to include <system_error>, which pulls in <string> -- a header which the <charconv> proposal intends to keep away from.
Reviewers: mclow.lists, EricWF
Reviewed By: mclow.lists
Subscribers: christof, cfe-commits
Differential Revision: https://reviews.llvm.org/D41347