[Bug c++/54316] [C++11] move constructor for string_stream

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54316

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-08-18 
21:43:52 UTC ---
(In reply to comment #2)
> It's known.

And documented
http://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2011