A non-standard clock may tick more slowly than
std::chrono::steady_clock.  This means that we risk returning false
early when the specified timeout may not have expired. This can be
avoided by looping until the timeout time as reported by the
non-standard clock has been reached.

Unfortunately, we have no way to tell whether the non-standard clock
ticks more quickly that std::chrono::steady_clock. If it does then we
risk returning later than would be expected, but that is unavoidable and
permitted by the standard.

2019-12-02  Mike Crowe  <mac@mcrowe.com>

	PR libstdc++/91906 Fix timed_mutex::try_lock_until on arbitrary clock
	* include/std/mutex (__timed_mutex_impl::_M_try_lock_until): Loop
	until the absolute timeout time is reached as measured against the
	appropriate clock.
	* testsuite/util/slow_clock.h: New file. Move implementation of
	slow_clock test class.
	* testsuite/30_threads/condition_variable/members/2.cc: Include
	slow_clock from header.
	* testsuite/30_threads/shared_timed_mutex/try_lock/3.cc: Convert
	existing test to templated function so that it can be called with
	both system_clock and steady_clock.
	* testsuite/30_threads/timed_mutex/try_lock_until/3.cc: Also run test
	using slow_clock to test above fix.
	* testsuite/30_threads/recursive_timed_mutex/try_lock_until/3.cc:
	Likewise.
	* testsuite/30_threads/recursive_timed_mutex/try_lock_until/4.cc: Add
	new test that try_lock_until behaves as try_lock if the timeout has
	already expired or exactly matches the current time.

From-SVN: r278902