libstdc++/4150: catastrophic performance decrease in C++ code

[Benjamin Kosnik]

Here's a patch against mainline. Jason, 'make check-script' also
compares the generated output from the 27_io tests and diffs them
against known good output. Your patch changes filebuf_virtuals-3.txt,
but after looking at it, I think it might be evidence of the bug you
mentioned earlier. The other two files are ok.

Please let me know if this is the case, if so, I think you are good to
go, and you'll just have to modify filebuf_virtuals-3.txt. See the
attached diff.

> Hand tested some additional cases.  test03 in
> 27_io/narrow_stream_objects.cc(must be enabled and run interactively
> by hand) is now broken on this platform. But it may be a latent issue
> since the cin buffer size was always constrained to 1 on this
> platform.  Can you confirm that this case isn't broken for you with
> your patch?

With and without the patch, test03 has the same behavior on linux...

> > Any objections?
> 
> Only the concern I have is about the remaining failing interactive
> test case.  

...it was disabled because I don't think it ever worked.

Do you have that scott synder redirection test? I can never remember
what it is. (Which means it should really be checked in, I guess, even
though that also has to be run by hand.)

-benjamin
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