[llvm] [ADT] Implement EnumeratedArray with std::array (NFC) (PR #158407)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar approved this pull request.

Nice. I think the side-effect is that this now allows for 0-sized arrays. Could we add a test for this?

https://github.com/llvm/llvm-project/pull/158407