[libcxx-commits] [libcxx] [libc++][mdspan] Fix `layout_left` / `layout_right` conversion constraint (PR #192847)

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eiytoq wrote:

`is_constructible_v<extents_type, _OtherExtents>` requires `_OtherExtents::rank() == extents_type::rank()`, so `_OtherExtents::rank() <= 1` is equivalent to checking `extents_type::rank() <= 1`. This is not an observable change.

https://github.com/llvm/llvm-project/pull/192847