libstdc++: Better CTAD for span and mdspan [PR120914].

This implements P3029R1. In P3029R1, the CTAD for span is refined to
permit deducing the extent of the span from an integral constant, e.g.

  span((T*) ptr, integral_constant<size_t, 5>{});

is deduced as span<T, 5>. Similarly, in

  auto exts = extents(integral_constant<int, 2>);
  auto md = mdspan((T*) ptr, integral_constant<int, 2>);

exts and md have types extents<size_t, 2> and mdspan<double,
extents<size_t, 2>>, respectively.

	PR libstdc++/120914

libstdc++-v3/ChangeLog:

	* include/std/span (span): Update CTAD to enable
	integral constants [P3029R1].
	* include/std/mdspan (extents): ditto.
	(mdspan): ditto.
	* testsuite/23_containers/span/deduction.cc: Test deduction
	guide.
	* testsuite/23_containers/mdspan/extents/misc.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>

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