US170 17.11 [cmp] Remove strong_equality and weak_equality

[wg21bot]

The strong_equality and weak_equality comparison categories don’t make sense now that we split equality from ordering. It doesn’t make sense to declare an operator<=> that returns one of these – they just add needless complexity.

Proposed change:
Remove strong_equality and weak_equality. Simplify three_way_comparable{,_with} to only deal with the ordering categories.

[brevzin]

Paper attached to LEWG wiki: http://wiki.edg.com/pub/Wg21belfast/LibraryEvolutionWorkingGroup/d1959r0.html

[tituswinters]

LEWG in Belfast: Forward D1959R0 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.)

[mclow]

LWG reviewed the library wording in this paper, and approved on Thursday afternoon.

[jensmaurer]

CWG in Belfast: Accepted.