[libc++][chrono] Avoid tautological comparisions.

In our implementation the year is always less than or equal to the
class' `max()`. It's unlikely this ever changes since changing the
year's range will be an ABI break. A static_assert is added as a
guard.

This was reported by @philnik.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D129442