libstdc++: Check prerequisites of layout_*::operator().

Previously, the prerequisite that the arguments passed to operator() are
a multi-dimensional index (of extents()) was not checked.

Both mapping::operator() and mdspan::operator[] have the same
prerequisite. Since, mdspan must check the prerequisite for user-defined
layout mappings, the preference is to check in mdspan.

Because out-of-bounds accesses are very common it's nevertheless useful
to check the prerequisite in mapping::operator(). This is relevant for
cases where the layout mappings are used without mdspan. This commit
checks the prerequisites via _GLIBCXX_DEBUG_ASSERTs and adds the required
tests.

More discussion in the email chain starting at:

  https://gcc.gnu.org/pipermail/libstdc++/2025-July/062265.html

libstdc++-v3/ChangeLog:

	* include/std/mdspan: Check prerequisites of
	layout_*::operator() with _GLIBCXX_DEBUG_ASSERTs.
	* testsuite/23_containers/mdspan/layouts/debug/out_of_bounds_neg.cc:
	Add tests for prerequisites.

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

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