FR: Add backport for std::clamp

The cool bit about it compared to juce::jlimit is that it can be used in constexpr circumstances. Worth noting that std::clamp is only available in C++17.

// Somewhere in juce_CompilerSupport.h:

#if ! JUCE_CXX17_IS_AVAILABLE

namespace std
{
    template<class T>
    constexpr const T& clamp (const T& v, const T& lo, const T& hi)
    {
        return (v < lo) ? lo : (hi < v) ? hi : v;
    }

    template<class T, class Compare>
    constexpr const T& clamp (const T& v, const T& lo, const T& hi, Compare comp)
    {
        return comp (v, lo) ? lo : comp (hi, v) ? hi : v;
    }
}

#endif