libstdc++: Make layout_left(layout_stride) noexcept.

libstdc++: Make layout_left(layout_stride) noexcept.

[mdspan.layout.left.cons] of N4950 states that this ctor is not
noexcept. Since, all other ctors of layout_left, layout_right or
layout_stride are noexcept, the choice was made, based on
[res.on.exception.handling], to make this ctor noexcept.

Two other major standard library implementations make the same choice.

libstdc++-v3/ChangeLog:

	* include/std/mdspan (layout_left): Strengthen the exception
	guarantees of layout_left::mapping(layout_stride::mapping).
	* testsuite/23_containers/mdspan/layouts/ctors.cc:
	Simplify tests to reflect the change.

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