PR libstdc++/58038
	PR libstdc++/60421
	* include/std/thread (this_thread::sleep_for): Check for negative
	durations.
	(this_thread::sleep_until): Check for times in the past.
	* testsuite/30_threads/this_thread/58038.cc: New.
	* testsuite/30_threads/this_thread/60421.cc: New.

From-SVN: r221708