libstdc++: Simplify construction of comparison category types

libstdc++: Simplify construction of comparison category types

The _Eq and _Ord enumerations can be combined into one, reducing the
number of constructors needed for the comparison category types. The
redundant equal enumerator can be removed and equivalent used in its
place. The _Less and _Greater enumerators can be renamed because 'less'
and 'greater' are already reserved names anyway.

	* libsupc++/compare (__cmp_cat::_Eq): Remove enumeration type.
	(__cmp_cat::_Ord::equivalent): Add enumerator.
	(__cmp_cat::_Ord::_Less, __cmp_cat::_Ord::_Greater): Rename to less
	and greater.
	(partial_ordering, weak_ordering, strong_ordering): Remove
	constructors taking __cmp_cat::_Eq parameters. Use renamed
	enumerators.