[libcxx] adds concept `std::assignable_from`

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

Depends on D96660

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D96742

GitOrigin-RevId: 647af31e74830e067834f50985f18ac7cc16c2a3
2 files changed