libstdc++: Implement std::layout_right_padded [PR110352].

libstdc++: Implement std::layout_right_padded [PR110352].

This commit adds the right padded layout as described in N5014, with
LWG4372 (dynamic padding value) and LWG4314 (move in operator()).

	PR libstdc++/110352

libstdc++-v3/ChangeLog:

	* include/std/mdspan (_RightPaddedIndices): Traits for right
	padded layouts.
	(layout_right::mapping::mapping) New overload for right padded
	layouts.
	(layout_right_padded): Add implementation.
	* src/c++23/std.cc.in (layout_right_padded): Add.
	* testsuite/23_containers/mdspan/layouts/ctors.cc: Update
	test for right padded layouts.
	* testsuite/23_containers/mdspan/layouts/empty.cc: Ditto.
	* testsuite/23_containers/mdspan/layouts/mapping.cc: Ditto.
	* testsuite/23_containers/mdspan/layouts/padded.cc: Ditto.
	* testsuite/23_containers/mdspan/layouts/padded_neg.cc: Ditto.
	* testsuite/23_containers/mdspan/layouts/padded_traits.h: Ditto.

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