[libc++] Simplify allocator propagation in vector assignments (#217635)

We can just use `if constexpr` instead of dispatching based on type
traits. This will cause some more code generation in C++03, but I don't
think we should care at this point.

GitOrigin-RevId: 7a1f6ad89e574d638e4618f41f243c63da76ce7b