[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
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 4f4effd6ee074700ca72182f7b38976f0e231ea6
diff --git a/include/module.modulemap b/include/module.modulemap
index 5b3ebf4..70f82a7 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -482,6 +482,7 @@
   // FIXME: These should be private.
   module __bit_reference { header "__bit_reference" export * }
   module __debug { header "__debug" export * }
+  module __errc { header "__errc" export * }
   module __functional_base { header "__functional_base" export * }
   module __hash_table { header "__hash_table" export * }
   module __locale { header "__locale" export * }