[libcxx][ranges] Create a test tool `ProxyIterator` that customises `iter_move` and `iter_swap`

It is meant to be used in ranges algorithm tests.
It is much simplified version of C++23's tuple + zip_view.
Using std::swap would cause compilation failure and using `std::move` would not create the correct rvalue proxy which would result in copies.

Differential Revision: https://reviews.llvm.org/D129099