[libc++][variant] P2637R3: Member `visit` (`std::variant`) (#76447)

Implements parts of: `P2637R3` https://wg21.link/P2637R3
(https://eel.is/c++draft/variant.visit)

Implements:
`variant.visit()`
`variant.visit<R>()`

The tests are as close as possible to the non-member function.

To land after: https://github.com/llvm/llvm-project/pull/76268



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