[libc++][chrono] Adds operator<=> for day.

Since the calendar classes were introduced in C++20 there's no need to
keep the old comparison operators.

This commit does the day calender class, the other calendar classes will
be in a followup commit.

Implements parts of:
- P1614R2 The mothership has landed

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D128603