[libc++][span] P2821R5: `span.at()` (#74994)

- Implements: [P2821R5: span.at()](https://wg21.link/P2821R5)
  (https://eel.is/c++draft/views.contiguous#views.span

)
- Cleaned up `span.operator[]` test

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Co-authored-by: Zingam <zingam@outlook.com>