libstdc++: Fix constraint for custom integer types in mdspan [PR121061]

libstdc++: Fix constraint for custom integer types in mdspan [PR121061]

PR121061 consists of two bugs for mdspan related code. This commit fixes
the first one. Namely, when passing custom IndexType as an array or
span, the conversion to int must be const. Prior to this commit the
constraint incorrectly also allowed non-const conversion. This commit
updates all related constraints to check

  __valid_index_type<const OtherIndexType&, index_type>

in those cases. Also adds a MutatingInt to int_like.h which only
supports non-const conversion to int and updates the tests.

	PR libstdc++/121061

libstdc++-v3/ChangeLog:

	* include/std/mdspan (extents::extents): Fix constraint to
	prevent non-const conversion to index_type.
	(layout_stride::mapping::mapping): Ditto.
	(mdspan::mdspan): Ditto.
	(mdspan::operator[]): Ditto.
	* testsuite/23_containers/mdspan/extents/custom_integer.cc: Add
	test for MutatingInt.
	* testsuite/23_containers/mdspan/int_like.h (MutatingInt): Add.
	* testsuite/23_containers/mdspan/layouts/mapping.cc: Add test for
	MutatingInt.
	* testsuite/23_containers/mdspan/layouts/stride.cc: Ditto.
	* testsuite/23_containers/mdspan/mdspan.cc: Ditto.

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