re PR libstdc++/54562 (mutex and condition variable timers)

re PR libstdc++/54562 (mutex and condition variable timers)

	PR libstdc++/54562
	* include/std/mutex (__timed_mutex_impl::__clock_t): Use
	high_resolution_clock for absolute timeouts, because
	pthread_mutex_timedlock uses CLOCK_REALTIME not CLOCK_MONOTONIC.
	(__timed_mutex_impl::_M_try_lock_for): Use steady_clock for relative
	timeouts as per [thread.req.timing].
	(__timed_mutex_impl::_M_try_lock_until<Clock,Duration>): Convert to
	__clock_t time point instead of using _M_try_lock_for.

From-SVN: r204672