[libc++] [P1065] Constexpr invoke, reference_wrapper, mem_fn, not_fn, default_searcher.
This completes the implementation of P1065 "constexpr INVOKE":
This doesn't yet complete the implementation of P1032 "Misc constexpr bits,"
but it does complete all of the <functional> bits, which means
that we can now set __cpp_lib_constexpr_functional for C++20.
This could use more constexpr tests for std::reference_wrapper<T>,
but the existing tests are extremely non-constexpr-friendly and
so I don't want to get into that rabbit-hole today.
Differential Revision: https://reviews.llvm.org/D93815