PL161 16.05.1.3 [compliance] Make <compare> a freestanding header

[wg21bot]

<compare> is currently not a freestanding header. This causes two problems:

1. It is impossible to use the spaceship operator functionality in a minimal freestanding implementation.
2. The <coroutine> header, which is freestanding, uses <compare>, which is not.

Proposed change:
Adopt P1855. cplusplus/papers#609

[wg21bot]

Duplicate of RU009 #9.

[tituswinters]

LEWG in Belfast: Forward P1855 to LWG for C++20.
(Prioritization on this vs. other paper-sized NB comments will be shared with LWG chair when LEWG finishes NB comment processing.)

[dietmarkuehl]

LWG small group looked at that on Wednesday morning: making freestanding
seems to be the proper direction. Solved by p1855. Duplicate of US158.

[JeffGarland]

Reviewed and approved for C++20 by LWG during Wed issue processing in Belfast.

[jensmaurer]

Accepted. See paper P1855R0.