P1614R2 Adding Spaceship <=> To The Library

[StephanTLavavej]

P1614R2 Adding Spaceship <=> To The Library

LWG-3324 Special-case std::strong/weak/partial_order for pointers
LWG-3347 std::pair<T, U> now requires T and U to be less-than-comparable
LWG-3350 Simplify return type of lexicographical_compare_three_way
LWG-3354 has_strong_structural_equality has a meaningless definition
LWG-3380 common_type and comparison categories
LWG-3426 operator<=>(const unique_ptr<T, D>&, nullptr_t) can't get no satisfaction
LWG-3427 operator<=>(const shared_ptr<T>&, nullptr_t) definition ill-formed
LWG-3432 Missing requirement for comparison_category

Feature-test macro as of WG21-N4842 (note that WG21-P1614 proposed __cpp_lib_spaceship which was removed by WG21-P1902. Instead, __cpp_lib_three_way_comparison which was added for #64 WG21-P0768 "Library Support For The Spaceship Comparison Operator <=>" is being increased):
#define __cpp_lib_three_way_comparison 201907L

[CaseyCarter]

NOTE: This proposal's changes to common_type are unnecessary after application of P1959R0 "Remove std::weak_equality and std::strong_equality". We have test coverage to validate this, and have submitted LWG-3380 to revert P1614R2's change to common_type.