Incorrect derivative for while-loops with declarations and assignments in forward mode

[gojakuch]

In the following example, fn is basically a zero function. Yet, Clad computes its derivative to be 1.

double fn(double x) {
    while (double t = (x = 0)) {}
    return x;
} // = 0

int main() {
    auto f_dx = clad::differentiate(fn, 0);

    std::cout << f_dx.execute(5) << '\n';
}

Expected output: 0

Output: 1

Same thing for

double fn(double x) {
    while ((x = 0)) {}
    return x;
} // = 0

int main() {
    auto f_dx = clad::differentiate(fn, 0);

    std::cout << f_dx.execute(5) << '\n';
}

Expected output: 0

Output: 1

This could be solved similarly to how #905 tries to solve the same issue for for-loops, that is with comma operators with a possibility to support more complicated conditions like (y = x*x) || (y = y - 1).