[v3 PATCH] Implement LWG 2534, Constrain rvalue stream operators.

[David Edelsohn]

I believe that this broke g++.old-deja/g++.law/ctors10.C

invalid initialization of reference of type 'Klasse::Err&' from
expression of type 'std::basic_ostream<char>::__ostream_type {aka
std::basic_ostream<char>}

- David