libstdc++: Refactor mdspan tests [PR121061]

libstdc++: Refactor mdspan tests [PR121061]

PR121061 shows that the test coverage for custom integer types is
insufficient. Custom IndexTypes are passed to mdspan related objects in
one of two ways:

  * as a template parameter pack,
  * or as an array/span.

These two cases have different requirements on the (constness of) custom
IndexTypes. Therefore, the tests are restructured as follows:

  * allow testing with different custom integers,
  * separate code that tests the two cases described above,
  * use int_like.h for all tests with custom integers.

The affected tests are for:

  * creating extents, layout_stride::mapping and mdspan from
  custom integers,

  * mapping::operator() and mdspan::operator[].

	PR libstdc++/121061

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/mdspan/extents/custom_integer.cc:
	Enable checking with different custom integers. Improve
	checking non-existence of overloads for incompatible custom
	integers.
	* testsuite/23_containers/mdspan/layouts/mapping.cc: ditto. Also
	improve reuse of int_like.h.
	* testsuite/23_containers/mdspan/layouts/stride.cc: ditto.
	* testsuite/23_containers/mdspan/mdspan.cc: ditto.
	* testsuite/23_containers/mdspan/extents/int_like.h: Rename (old
	name).
	* testsuite/23_containers/mdspan/int_like.h: Rename (new name).
	(ThrowingInt): Add.
	(NotIntLike): Add.

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