[libc++] Investigate how we create iterators in bitset

[ldionne]

I am tempted to see whether we can introduce __begin() and __end() functions instead, and get rid of __make_iter entirely. I'd imagine something like

iterator __begin() { return /* __make_iter(0); */ }
iterator __end() { return /* __make_iter(_Size); */ }

// and then here's an example usage
// OLD:
std::copy_backward(__base::__make_iter(0), __base::__make_iter(_Size - __pos), __base::__make_iter(_Size));

// NEW:
std::copy_backward(__begin(), __end() - __pos, __end());

Now the main question IMO is whether __end() - __pos produces equivalent code. I think we'll end up calling

llvm-project/libcxx/include/__bit_reference

Line 368 in 52f941a

_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bit_iterator& operator+=(difference_type __n) {

, which might not produce efficient code. I still think we should investigate that since that looks like the way we'd want to write our code at a high level.

Originally posted by @ldionne in #121312 (comment)