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

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

Implements submdspan for layout_left_padded as described in P3663.

	PR libstdc++/110352

libstdc++-v3/ChangeLog:

	* include/std/mdspan (__mdspan::__is_padded_mapping): Define.
	(__mdspan::_SubMdspanMapping): Add _Padded template parameter.
	(__mdspan::_SubMdspanMapping<_LayoutSide::__left, true>): Define.
	(__mdspan::__submdspan_mapping_impl): Updated _Trait alias.
	(layout_left_padded::submdspan_mapping): New friend method.
	* testsuite/23_containers/mdspan/layout_traits.h
	(LayoutTraits::layout_same_padded): New template type alias.
	* testsuite/23_containers/mdspan/submdspan/selections/left_padded.cc:
	Instantiate tests for layout_left_padded.
	* testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc:
	Ditto.

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