I believe this is a complete implementation of P1065R2 "Constexpr INVOKE" (Tomasz Kamiński, Barry Revzin), which was adopted for C++20.
This could use more constexpr tests for std::reference_wrapper<T>, but the existing tests are extremely constexpr-unfriendly and so I don't want to get into that rabbit-hole today.
I plan to push the refactoring of ForwardingCallObject as a separate preliminary commit:
[libc++] Constexpr-proof some machinery in not_fn.pass.cpp. NFCI.