[v3] libstdc++/17922

[Benjamin Kosnik]

>I was referring to the problem we have with 
>_S_ios_openmode_end & co, which the user does *not* know and should 
>*not* trigger a warning if missing from the switch.

Oh, this. Yes, I don't know what to do about this issue.

For the moment, let's just leave this stuff in. I've marked __ios_flags
with GLIBCXX_ABI DEPRECATED so we'll remember at some point to take out
__ios_flags, and at that point the various _end_ enums can be removed.

thanks,
benjamin