libstdc++: Fix try_lock_until and try_lock_shared_until on arbitrary clock

libstdc++: Fix try_lock_until and try_lock_shared_until on arbitrary clock

This is the equivalent to PR libstdc++/91906, but for shared_mutex.

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 without
waking up periodically to check, which would be rather too expensive.

François Dumont pointed out[1] a flaw in an earlier version of this patch
that revealed a hole in the test coverage, so I've added a new test that
try_lock_until acts as try_lock if the timeout has already expired.

[1] https://gcc.gnu.org/ml/libstdc++/2019-10/msg00021.html

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

	Fix try_lock_until and try_lock_shared_until on arbitrary clock
	* include/std/shared_mutex (shared_timed_mutex::try_lock_until)
	(shared_timed_mutex::try_lock_shared_until): Loop until the absolute
	timeout time is reached as measured against the appropriate clock.
	* testsuite/30_threads/shared_timed_mutex/try_lock_until/1.cc: New
	file. Test try_lock_until and try_lock_shared_until timeouts against
	various clocks.
	* testsuite/30_threads/shared_timed_mutex/try_lock_until/1.cc: New
	file. Test try_lock_until and try_lock_shared_until timeouts against
	various clocks.

From-SVN: r278904