[libc++][chrono] Uses operator<=> in the calendar.

[Mordante]

Summary

Since the calendar is added in C++20 the existing operators are removed.

Implements part of:

• P1614R2 The Mothership has Landed

[ldionne]

libcxx/include/__chrono/year_month.h

55–60

Is this an omission?

libcxx/test/std/time/time.cal/time.cal.ym/time.cal.ym.nonmembers/comparisons.pass.cpp

43–52

Let's just always use 1010, I don't think we gain any fundamental coverage by doing more than that (do you agree?).

And in fact, we probably also want to test boundary values (at least year{0} and year{1}) if we don't already.

libcxx/test/std/time/time.cal/time.cal.ymd/time.cal.ymd.nonmembers/comparisons.pass.cpp

102

Same comment about number of steps and boundary values.

[Mordante]

Thanks for the review!

libcxx/include/__chrono/year_month.h

55–60

Yes, thanks for spotting the issue.

libcxx/test/std/time/time.cal/time.cal.ym/time.cal.ym.nonmembers/comparisons.pass.cpp

43–52

I'll change the range to -5 to 5. Same for the next one.

[Mordante]

Rebased and addresses the review comments.
I will land this after the CI is happy.