P0972R0 <chrono> zero(), min(), and max() should be noexcept

P0972R0 <chrono> zero(), min(), and max() should be noexcept

This paper has been included in the C++20 draft, but the changes to add
noexcept can be made unconditionally, to apply for C++11 too.

	* include/std/chrono (duration_values::zero(), duration_values::min())
	(duration_values::max()): Add noexcept.
	(duration::zero(), duration::min(), duration::max()): Likewise.
	(time_point::zero(), time_point::min(), time_point::max()): Likewise.
	* testsuite/20_util/duration/requirements/noexcept.cc: New test.
	* testsuite/20_util/time_point/requirements/noexcept.cc: New test.

From-SVN: r267865