mdspan: question about layout_stride constructors for layout_left and layout_right

[MattStephanson]

While reviewing <mdspan> for MSVC STL, @StephanTLavavej noticed that the layout_stride::mapping constructor is noexcept for layout_right::mapping, but not for layout_left::mapping. It appears this happened in #181. They both have similar preconditions, so we're not sure if there's a reason for it that's escaped us, or if it was a typo.

[dalg24]

This is an oversight. We will submit a defect report for it.