use auto for iterators

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/home/runner/work/cppcheck/cppcheck/lib/utils.h:412:16: error: returning a constant reference parameter may cause use-after-free when the parameter is constructed from a temporary [bugprone-return-const-ref-from-parameter,-warnings-as-errors]
  412 |         return t;
      |                ^

#include <set>
#include <type_traits>

namespace utils {
    template<class T>
    constexpr typename std::add_const<T>::type & as_const(T& t) noexcept
    {
        return t;
    }
}

struct C
{
    const int* f() const
    {
        const auto it = utils::as_const(v).find(0);
        if (it == v.cend())
            return nullptr;
        return &(*it);
    }

    std::set<int> v;
};

https://godbolt.org/z/91qxYYxcz

If you remove the const from the method the warning goes away.

This happens because we try as_const() something that is already const. That is intentional though.

It seems like this might just be a false positive since everything seems to work fine and the CI is happy otherwise.

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These changes exposed a unreadVariable false negative: https://trac.cppcheck.net/ticket/13303.

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This is ready for review but keeping a draft so it does not interfere with the ValueFlow revert.