[v3] Fix second half of libstdc++/6745

[Jonathan Lennox]

On Monday, November 18 2002, "Paolo Carlini" wrote to "Jonathan Lennox, gcc-patches@gcc.gnu.org, libstdc++@gcc.gnu.org" saying:

> I want to study this in more detail but basically I believe you are 
> right: we have a performance issue in such cases. Could you possibly work
> out a patch along the lines which you suggested?
> Not adding new public members to basic_filebuf, however...

Is making __copy_streambufs a friend of basic_filebuf acceptable?

Also, I'm still not clear on whether it's acceptable to use dynamic_cast<>
in libstdc++.

-- 
Jonathan Lennox
lennox at cs dot columbia dot edu