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

[James E T Smith via cfe-commits]

jamesETsmith wrote:

@philnik777, I think you're right that `Proxy` is missing a few member functions (and type aliases). Here's a minimal example of my additions to it: https://godbolt.org/z/8T9eqjjEj. If that seems reasonable, I'll update `test_iterators.h`.

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