Fix open_memstream namespace (bug 18498)

[Roland McGrath]

> 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.


Thanks,
Roland