[libc++][memory] P2652R2: Disallow Specialization of `allocator_traits` (#79978)

Implements P2652R2 <https://wg21.link/P2652R2>:
- https://eel.is/c++draft/allocator.requirements.general
- https://eel.is/c++draft/memory.syn
- https://eel.is/c++draft/allocator.traits.general
- https://eel.is/c++draft/allocator.traits.members
- https://eel.is/c++draft/diff.cpp20.concepts
- https://eel.is/c++draft/diff.cpp20.utilities

---------

Co-authored-by: Zingam <zingam@outlook.com>
GitOrigin-RevId: 7d78ccf7d5a9861839126d5899b443f55236cab9