[libcxx] adds concept `std::convertible_to`

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

Differential Revision: https://reviews.llvm.org/D77961

GitOrigin-RevId: 8caf835337f7bae9d686afdfc16c4897f2da61d8
2 files changed