Fix #1376 - Allow redefining builtin custom derivatives

[Lucky-Lodhi2004]

I made changes for only one derivative i.e. exp_pushforward. Changes for other derivatives can be made after discussing from maintainers.

Users can redefine builtin derivatives after my changes.

 ~/repos/llvm-project/build/bin/clang++ -std=c++17 -I ./include/ -fplugin=./build/lib/clad.so example.cpp -o example
./example
The code is: 
void differentiable_code_grad(double x, double *_d_x) {
    {
        double _r0 = 0.;
        _r0 += 1 * clad::custom_derivatives::std::exp_pushforward(x, 1.).pushforward;
        *_d_x += _r0;
    }
}

Changes

Added control macro for the definition of std::exp_pushforward.
here

// 2.1 exp, expf, expl
// Allow disabling built-in derivative if user-defined derivative provided.
#ifndef CLAD_NO_BUILTIN_EXP
template <typename T, typename dT>
CUDA_HOST_DEVICE ValueAndPushforward<T, dT> exp_pushforward(T x, dT d_x) {
  return {::std::exp(x), ::std::exp(x) * d_x};
}
#endif

and here:

#ifndef CLAD_NO_BUILTIN_EXP
using std::exp_pushforward;  //disable built-in exp derivative
#endif

Similar macro for all the functions can be used (if maintainers approve).