Fix warning C26817 on copying in `for (auto vh : value_and_holder(...))` (#2382)

* Fix warning C26817: Potentially expensive copy of variable 'vh' in range-for loop. Consider making it a const reference (es.71).

* Replace another instance of `for (auto vh : values_and_holders(...))` with `auto vh &` (found by @bstaletic)

Co-authored-by: Michael Goulding <Michael.Goulding@microsoft.com>
Co-authored-by: Yannick Jadoul <yannick.jadoul@belgacom.net>