[libc++][span] P2447R4: `std::span` over an initializer list (#78157)

Implements: https://wg21.link/P2447R6
- https://eel.is/c++draft/span.syn
- https://eel.is/c++draft/span.overview
- https://eel.is/c++draft/span.cons
- https://eel.is/c++draft/diff

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Co-authored-by: Zingam <zingam@outlook.com>
GitOrigin-RevId: dbbeee6b8357c5a68543f612f3b2b607f1911b4c