[libcxx] adds remaining callable concepts

* `std::predicate`
* `std::relation`
* `std::equivalence_relation`
* `std::strict_weak_order`

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/D96477