libstdc++: Validate user-provided stride values for layout_stride.

Converting the __strides values using __index_type_cast asserts
that they are non-negative and each value is representable as
index_type.

libstdc++-v3/ChangeLog:

	* include/std/mdspan
	(layout_stride::mapping::mapping(const extent_type&, span<...>)):
	Convert strides using __index_type_cast, which bring asserts
	for negative and unrepresentable values.
	* testsuite/23_containers/mdspan/layouts/stride_neg.cc: New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>