[llvm] [libc++] Implement ranges::iota (PR #68494)

[James E T Smith via llvm-commits]

jamesETsmith wrote:

The strategy using `ProxyDiffTBase` is implemented in 88e271a. Initially, it caused a bunch of errors elsewhere because the compiler generated `==` and `<=>` member functions would cause checks like `std::totally_ordered_with<Proxy<int&>, Proxy<int>&>` to fail (see https://godbolt.org/z/nqMn1jhYx). To address that I just implemented those functions, I hope that's ok.

I noticed that `test_iterators.h` isn't formatted (at least not with clang-format 17) so I opted not to format that file after making changes to keep the diffs clearer. I can format that as soon as we're done tinkering in it.

https://github.com/llvm/llvm-project/pull/68494