[Bug libstdc++/53683] cout doesn't support std::u16string

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53683

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Ed Smith-Rowland from comment #4)
> It looks like libcpp/charset.c might have a lot of the guts that could help
> with the implementation of codecvt_utf8, etc.
> 
> Maybe we could either get libcpp to handle it or just steal the code.

That would be useful, thanks for the pointer.  I have implementations of
wstring_convert and wbuffer_convert but no way to test them without the new
codecvt facets.