libstdc++: Implement ranges::fold_* from P2322R6

libstdc++: Implement ranges::fold_* from P2322R6

libstdc++-v3/ChangeLog:

	* include/bits/ranges_algo.h: Include <optional> for C++23.
	(__cpp_lib_fold): Define for C++23.
	(in_value_result): Likewise.
	(__detail::__flipped): Likewise.
	(__detail::__indirectly_binary_left_foldable_impl): Likewise.
	(__detail::__indirectly_binary_left_foldable): Likewise.
	(___detail:__indirectly_binary_right_foldable): Likewise.
	(fold_left_with_iter_result): Likewise.
	(__fold_left_with_iter_fn, fold_left_with_iter): Likewise.
	(__fold_left_fn, fold_left): Likewise.
	(__fold_left_first_with_iter_fn, fold_left_first_with_iter):
	Likewise.
	(__fold_left_first_fn, fold_left_first): Likewise.
	(__fold_right_fn, fold_right): Likewise.
	(__fold_right_last_fn, fold_right_last): Likewise.
	* include/std/version (__cpp_lib_fold): Likewise.
	* testsuite/25_algorithms/fold_left/1.cc: New test.
	* testsuite/25_algorithms/fold_right/1.cc: New test.