libstdc++: Fix split_view::_OuterIter::operator++ [LWG 3505]

libstdc++-v3/ChangeLog:

	* include/std/ranges (__detail::find): Define.
	(split_view::_OuterIter::operator++): Apply proposed resolution
	of LWG 3505.
	* testsuite/std/ranges/adaptors/split.cc (test10): New test.