[libcxx] adds concept std::semiregular

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

Depends on D97443

Reviewed By: Quuxplusone, EricWF, #libc

Differential Revision: https://reviews.llvm.org/D97911