[libcxx] adds [concepts.arithmetic]

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/D88131

GitOrigin-RevId: bee7b07f230d53de4945769a93490e3488252a7e
4 files changed