Fix testsuite failures caused by the patch implementing LWG 2534.

* include/std/istream (__is_convertible_to_basic_istream):
Change the return types of __check, introduce istream_type.
(operator>>(_Istream&&, _Tp&&)):
Use __is_convertible_to_basic_istream::istream_type as the return type.
* include/std/ostream (__is_convertible_to_basic_ostream):
Change the return types of __check, introduce ostream_type.
(operator>>(_Ostream&&, _Tp&&)):
Use __is_convertible_to_basic_ostream::ostream_type as the return type.

From-SVN: r243036