[libcxx] adds concept std::copyable

Implements parts of:
    - P0898R3 Standard Library Concepts
    - P1754 Rename concepts to standard_case for C++20, while we still can

Depends on D97359

Reviewed By: EricWF, #libc, Quuxplusone, zoecarver

Differential Revision: https://reviews.llvm.org/D97443

GitOrigin-RevId: 8ef69c66d5aab9ca2f04e0aecf109a2d2f7bb4b9
3 files changed