[Bug libstdc++/85343] New: Overload __throw_ios_failure to allow passing errno

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85343

            Bug ID: 85343
           Summary: Overload __throw_ios_failure to allow passing errno
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

The standard says "When throwing ios_base::failure exceptions, implementations
should provide values of ec that identify the specific reason for the failure."

We currently always use an error_code containing io_errc::stream.

We can probably only expect errno to be set for filebuf operations, and in
practice only for failed reads. Failed writes from basic_filebuf::overflow
return -1 which might cause a stream object to set failbit and then throw, but
at that point we're too far away from the write to know if errno can be relied
on or not.