[libc++][numeric] Modernized `std::midpoint` (#174596) - Some improvements to `std::midpoint` implementation: replaces `enable_if` with concepts. https://wg21.link/numeric.ops.midpoint Towards #172124 --------- Co-authored-by: Hristo Hristov <[email protected]> NOKEYCHECK=True GitOrigin-RevId: 6c2c0b9da2b96e06048d52b4308636e667e08cf2