* 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
