libstdc++ Simplify definition of net::socket_base constants

libstdc++ Simplify definition of net::socket_base constants

libstdc++-v3/ChangeLog:

	* include/experimental/socket (socket_base::shutdown_type):
	(socket_base::wait_type, socket_base::message_flags):
	Remove enumerators. Initialize constants directly with desired
	values.
	(socket_base::message_flags): Make all operators constexpr and
	noexcept.
	* testsuite/util/testsuite_common_types.h (test_bitmask_values):
	New test utility.
	* testsuite/experimental/net/socket/socket_base.cc: New test.