Clad crashes when a function has a default object argument

[gojakuch]

it doesn't matter if the argument;s default value is overwritten with another value or not -- Clad crashes anyway. it's important to prevent Clad from crashing in this case even if we don't support the default arguments in general yet (#1028), since some STL (#1067) and Kokkos functionality depends on this.

reproducer:

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

#define show(x) std::cout << #x << ": " << x << "\n";

double g(double x, const std::string &s = "default string") {
    return x;
}

double fn(double u, double v) {
    double x = g(u);
    return x;
}

int main() {
    auto d_fn = clad::gradient(fn);
    double u = 3, v = 5;
    double du, dv;
    du = dv = 0;
    show(fn(u, v));
    d_fn.execute(u, v, &du, &dv);
    show(du);
    show(dv);
}