[llvm] ADT/Matrix: two-dimensional Container with View (PR #98893)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar requested changes to this pull request.

Why would we want to have a dedicated owning (storage) type for 2d arrays? If all you want is to support 2d indexing, I'd imagine something like `std::ndspan` over a plain allocation (e.g., vector) would suffice?

Also, how is this related to adding `constexpr` to array ref? I understand it's the base PR, but why do we need `constexpr` in the first place?

https://github.com/llvm/llvm-project/pull/98893