libstdc++: Fix <stop_token> and improve tests

libstdc++: Fix <stop_token> and improve tests

	* include/std/stop_token: Reduce header dependencies by including
	internal headers.
	(stop_token::swap(stop_token&), swap(stop_token&, stop_token&)):
	Define.
	(operator!=(const stop_token&, const stop_token&)): Fix return value.
	(stop_token::_Stop_cb::_Stop_cb(Cb&&)): Use std::forward instead of
	(stop_token::_Stop_state_t) [_GLIBCXX_HAS_GTHREADS]: Use lock_guard
	instead of unique_lock.
	[!_GLIBCXX_HAS_GTHREADS]: Do not use mutex.
	(stop_token::stop_token(_Stop_state)): Change parameter to lvalue
	reference.
	(stop_source): Remove unnecessary using-declarations for names only
	used once.
	(swap(stop_source&, stop_source&)): Define.
	(stop_callback(const stop_token&, _Cb&&))
	(stop_callback(stop_token&&, _Cb&&)): Replace lambdas with a named
	function. Use std::forward instead of std::move. Run callbacks if a
	stop request has already been made.
	(stop_source::_M_execute()): Remove.
	(stop_source::_S_execute(_Stop_cb*)): Define.
	* include/std/version (__cpp_lib_jthread): Define conditionally.
	* testsuite/30_threads/stop_token/stop_callback.cc: New test.
	* testsuite/30_threads/stop_token/stop_source.cc: New test.
	* testsuite/30_threads/stop_token/stop_token.cc: Enable test for
	immediate execution of callback.

From-SVN: r278325