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
