[libcxx] adds concepts `std::invocable` and `std::regular_invocable`

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/D96235

GitOrigin-RevId: 17db24a7a8f23eb5adb24f36b9d51bb7cc5ae564
5 files changed