libstdc++: Give split_view::_Sentinel a default ctor [PR101214]

libstdc++: Give split_view::_Sentinel a default ctor [PR101214]

This gives the new split_view's sentinel type a defaulted default
constructor, something which was overlooked in r12-1665.  This patch
also fixes a couple of other issues with the new split_view as reported
in the PR.

	PR libstdc++/101214

libstdc++-v3/ChangeLog:

	* include/std/ranges (split_view::split_view): Use std::move.
	(split_view::_Iterator::_Iterator): Remove redundant
	default_initializable constraint.
	(split_view::_Sentinel::_Sentinel): Declare.
	* testsuite/std/ranges/adaptors/split.cc (test02): New test.