Implement LWG 2534, Constrain rvalue stream operators.

* include/std/istream (__is_convertible_to_basic_istream): New.
(__is_extractable): Likewise.
(operator>>(basic_istream<_CharT, _Traits>&&, _Tp&&)):
Turn the stream parameter into a template parameter
and constrain.
* include/std/ostream (__is_convertible_to_basic_ostream): New.
(__is_insertable): Likewise.
(operator<<(basic_ostream<_CharT, _Traits>&&, const _Tp&)):
Turn the stream parameter into a template parameter
and constrain.
* testsuite/27_io/basic_istream/extractors_other/char/4.cc: New.
* testsuite/27_io/basic_istream/extractors_other/wchar_t/4.cc:
Likewise.
* testsuite/27_io/basic_ostream/inserters_other/char/6.cc: Likewise.
* testsuite/27_io/basic_ostream/inserters_other/wchar_t/6.cc: Likewise.

From-SVN: r243006