[libcxx] adds common_reference to <type_traits>

Implements part of P0898R3 Standard Library Concepts

Reworks D74351 to use requires-clauses over SFINAE and so that it more
closely follows the wording.

Co-authored by: Michael Schellenberger Costa <mschellenbergercosta@googlemail.com>

(Michael did all the heavy lifting and I came in to polish it for
 submission, since Michael is focussing on `std::format` now.)

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D96657