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

[MichaelGoulding]

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

EDIT (@YannickJadoul): Fixes #2379

[YannickJadoul]

value_and_holder shouldn't be too expensive to copy (and quite optimizable, even, I'd think?), but I see no immediate reason why this wouldn't work, so why not! 👍

[bstaletic]

In theory, value_and_holder is big enough to be passed by reference. In practice, gcc and clang seem to output the same assembly.

Besides the auto vh in cast.h, there's also one in detail/class.h. We should fix both.

[YannickJadoul]

Good catch, @bstaletic

[YannickJadoul]

I've made those changes and pushed them. Let's see if CI turns nicely green.

[YannickJadoul]

Green, green, green, green, green, ...

Thanks for reporting and fixing, @MichaelGoulding!

[MichaelGoulding]

Thanks, for fixing up the last bit. @YannickJadoul