Clad generates noncompilable code for operators in the reverse mode

[gojakuch]

#include "clad/Differentiator/Differentiator.h"
#include "clad/Differentiator/STLBuiltins.h"
#include <iostream>

double fn(double x, double y) {
    std::vector<double> a;
    a.push_back(x);
    a[0] *= x;
    return a[0];
}

int main(int argc, char* argv[]) {
    double dx, dy;
    auto df = clad::gradient(fn, "x, y");
    std::cout << fn(3, 4) << '\n';
    dx = 0; dy = 0;
    df.execute(3, 4, &dx, &dy);
    std::cout << dx << ' ' << dy << '\n';
}

in the generated code for the line a[0] *= x clad uses a variable that is only created during the reverse pass later on.

the reason for this has been mentioned in #1076. this issue is related to #1066 which is the same problem, but for constructors and not operators.