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

[Jakub Kuderski via llvm-commits]

kuhar wrote:

> > Nice. I think the side-effect is that this now allows for 0-sized arrays. Could we add a test for this?
> 
> @kuhar I added `static_assert` to disallow 0-sized arrays.

Why not allow them?

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