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
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@336164 91177308-0d34-0410-b5e6-96231b3b80d8
