[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