libstdc++: P1964R2 Wording for boolean-testable

libstdc++: P1964R2 Wording for boolean-testable

This removes the complicated std::boolean concept, as agreed in Prague.

	* include/bits/ranges_algo.h (__find_fn, __find_first_of_fn)
	(__adjacent_find_fn): Cast result of predicate to bool.
	* include/std/concepts (__boolean): Remove.
	(__detail::__boolean_testable_impl, __detail::__boolean_testable): Add
	new helper concepts.
	(__detail::__weakly_eq_cmp_with, totally_ordered, totally_ordered_with)
	(predicate): Use __boolean_testable instead of boolean.
	* libsupc++/compare (__detail::__partially_ordered, _Synth3way):
	Likewise.