Fix open_memstream namespace (bug 18498)

[Joseph Myers]

On Sat, 6 Jun 2015, Roland McGrath wrote:

> > > The C++ code is in testcases, which produce an "expected initializer 
> > > before 'throw'" error if you use the typeof construct (that is, C++ throw 
> > > () only works with an actual function declarator, not when typeof is used 
> > > to give something a function type).  Hence the comment, to explain why 
> > > __typeof isn't used.
> > 
> > OK.  It would not have looked so odd to me if the comment said "C++ tests".
> > Perhaps also some sort of XXX/TODO comment about how this should go away
> > when we one day stop building tests with the internal headers.
> 
> Or the decl could just use typeof and be under #ifdef _LIBC, no?

I've made the comment say "C++ tests".  This matches what we do in 
wchar.h.  As far as I know we still define _LIBC when building tests, as 
well as using internal headers for them.

-- 
Joseph S. Myers
joseph@codesourcery.com