[libc++][spaceship] Implement `operator<=>` for `time_point`

Depends on D145881

Implements parts of **P1614R2**: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1614r2.html
Implements `operator<=>` for `time_point`

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D146250