Spaceship CPO wording in [cmp.alg] needs an overhaul

[StephanTLavavej]

WG21-N4861's spaceship customization point objects are specified in [cmp.alg] with an older form of wording. There are a couple of things that should be cleaned up:

• The CPOs should pass arguments as lvalues.
• There should be clear wording to block unqualified name lookup when activating ADL.

Found while developing #1370.

[StephanTLavavej]

#3209 P2602R2 "Poison Pills Are Too Toxic" has overhauled the ADL wording, although I think the "should pass arguments as lvalues" part may still need an issue.