libstdc++: Implement submdspan_extents. [PR110352]

libstdc++: Implement submdspan_extents. [PR110352]

Implement submdspan_extents as described in P3663 and adds it to the std
module.

	PR libstdc++/110352

libstdc++-v3/ChangeLog:

	* include/std/mdspan (submdspan_extents): New function.
	* src/c++23/std.cc.in: Add submdspan_extents.
	* testsuite/23_containers/mdspan/int_like.h: Add StructuralInt.
	* testsuite/23_containers/mdspan/submdspan/submdspan_extents.cc: New test.
	* testsuite/23_containers/mdspan/submdspan/submdspan_extents_neg.cc: New test.

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