[libc++] Rename __identity to __type_identity

In C++20 the type trait `type_identity` was introduced. For the same purpose there is `__identity` for pre-C++20 code. The name is confusing, because since C++20 there is also `identity`, which isn't a type trait.

Reviewed By: ldionne, Mordante, #libc

Spies: EricWF, libcxx-commits

Differential Revision: https://reviews.llvm.org/D122017