[libc++] Fix <charconv> not exporting std::errc (#146268)

`<charconv>` doesn't properly export `std::errc` with locales disabled,
which causes the CI to fail. This fixed the modulemap to let
`from_chars_result` export `std::errc`, like it's supposed to do.

Author: philnik777

libcxx/include/module.modulemap.in

@@ -934,7 +934,10 @@ module std [system] {
     module chars_format               { header "__charconv/chars_format.h" }
     module from_chars_floating_point  { header "__charconv/from_chars_floating_point.h" }
     module from_chars_integral        { header "__charconv/from_chars_integral.h" }
-    module from_chars_result          { header "__charconv/from_chars_result.h" }
+    module from_chars_result          {
+      header "__charconv/from_chars_result.h"
+      export std.system_error.errc
+    }
     module tables                     { header "__charconv/tables.h" }
     module to_chars                   { header "__charconv/to_chars.h" }
     module to_chars_base_10           { header "__charconv/to_chars_base_10.h" }