[libc++][tuple][utility] P2968R2: Make `std::ignore` a first-class object (#97401)

Implements:  https://wg21.link/P2968R2

References:
- https://eel.is/c++draft/tuple.general
- https://eel.is/c++draft/tuple.syn
- https://eel.is/c++draft/tuple.creation
- https://github.com/cplusplus/draft/milestone/31
- https://github.com/cplusplus/draft/pull/7109
- https://github.com/cplusplus/papers/issues/1640
- https://cplusplus.github.io/LWG/issue2933
- https://cplusplus.github.io/LWG/issue3978

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Co-authored-by: Hristo Hristov <zingam@outlook.com>
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GitOrigin-RevId: 31c9c41873d06f3029ad200c04819e2e9d33c7e7
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