[libcxx][type_traits] remove `std::is_literal_type` and `std::result_of` for C++20

C++17 deprecated `std::is_literal_type` and `std::result_of`, C++20 removed them.

Implements parts of:
    * P0174R2 'Deprecating Vestigial Library Parts in C++17'.
    * P0619R4 'Reviewing Deprecated Facilities of C++17 for C++20'.

Reviewed By: ldionne, Mordante, Quuxplusone, #libc

Differential Revision: https://reviews.llvm.org/D102992