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

[mclow.lists]

Summary

This paper is up for a vote in San Diego next week.

Note: I marked the calls as noexcept all the time, not just for C++20 - because we have freedom to do that.

[EricWF]

Note: I marked the calls as noexcept all the time, not just for C++20 - because we have freedom to do that.
However, the tests just check when we're building for C++20.

I think this should be subject to the Beyonce rule: If you like the behavior, put a test on it.
I think we should use LIBCPP_ASSERT_NOEXCEPT to test our behavior pre-c++2a.

Other then that, this patch LGTM.

[ldionne]

LGTM too, but I agree w/ Eric regarding the tests. Also, the link to p0927r2 doesn't seem right.

[mclow.lists]

Added libc++-specific tests. Fixed link in summary.

[mclow.lists]

Committed revision 346766