[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

llvm-svn: 336164