libstdc++: Optimize C++20 comparison category types

libstdc++: Optimize C++20 comparison category types

This reduces the size and alignment of all three comparison category
types to a single byte. The partial_ordering::_M_is_ordered flag is
replaced by the value 0x02 in the _M_value member.

This also optimizes conversion and comparison operators to avoid
conditional branches where possible, by comparing _M_value to constants
or using bitwise operations to correctly handle the unordered state.

	* libsupc++/compare (__cmp_cat::type): Define typedef for underlying
	type of enumerations and comparison category types.
	(__cmp_cat::_Ord, __cmp_cat::_Ncmp): Add underlying type.
	(__cmp_cat::_Ncmp::unordered): Change value to 2.
	(partial_ordering::_M_value, weak_ordering::_M_value)
	(strong_ordering::_M_value): Change type to __cmp_cat::type.
	(partial_ordering::_M_is_ordered): Remove data member.
	(partial_ordering): Use second bit of _M_value for unordered. Adjust
	comparison operators.
	(weak_ordering::operator partial_ordering): Simplify to remove
	branches.
	(operator<=>(unspecified, weak_ordering)): Likewise.
	(strong_ordering::operator partial_ordering): Likewise.
	(strong_ordering::operator weak_ordering): Likewise.
	(operator<=>(unspecified, strong_ordering)): Likewise.
	* testsuite/18_support/comparisons/categories/partialord.cc: New test.
	* testsuite/18_support/comparisons/categories/strongord.cc: New test.
	* testsuite/18_support/comparisons/categories/weakord.cc: New test.