LWG-3953 iter_move for common_iterator and counted_iterator should return decltype(auto)

As far as I know, the wording changes in this LWG issue don't have normative effects, and implementations are not actually required to make any change. However, per the description in the LWG issue, it seems better to switch to use decltype(auto) for the sake of compiler throughput.

Code:

You can just change iter_rvalue_reference_t<_Iter> to decltype(auto) in these two lines.

This issue is intended for a new contributor (especially one new to GitHub) to get started with the simplest possible change.

Please feel free to submit a pull request if there isn't one already linked here - no need to ask for permission! 😸

You can (and should) link your pull request to this issue using GitHub's close/fix/resolve syntax. (in the PR description not the commit message)