libstdc++: Implement submdspan_mapping for layout_right. [PR110352]

libstdc++: Implement submdspan_mapping for layout_right. [PR110352]

Adds submdspan_mapping for layout_right as described in P3663.

	PR libstdc++/110352

libstdc++-v3/ChangeLog:

	* include/std/mdspan
	(__mdspan::_SubMdspanMapping<_LayoutSide::__right>): Define.
	(layout_right::mapping::submdspan_mapping): New friend function.
	* testsuite/23_containers/mdspan/submdspan/selections/right.cc:
	Instantiate tests for layout_right.
	* testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc:
	Ditto.
	* testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc:
	Ditto.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>