[libcxx] adds concept std::regular

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

Depends on D97911

Reviewed By: EricWF, #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D98154

GitOrigin-RevId: 8d4af1b6e033fc6eba52d862cd93dc8939fea916
2 files changed