[c++17] [clang] force Clang to run constexpr function at compile time

reference:
C++17 constexpr everything (or as much as the compiler can)

#include <iostream>

constexpr int factorial(int n) {
     return n <= 1 ? 1 : (n * factorial(n - 1));
}

constexpr int fibonacci(unsigned n) {
    return n <= 1 ? n : fibonacci(n - 1) + fibonacci(n - 2);
}

int main() {
    std::cout << factorial(5) << '\n';
    std::cout << fibonacci(10) << '\n';
}

to:

int main() {
    constexpr int a = factorial(12);
    constexpr int b = fibonacci(10);
  
    // Check if the values are actually calculated at compile time
   static_assert(a == 479001600, "factorial failed\n");
   static_assert(b == 55, "fibonacci failed\n");
     
   std::cout << a << '\n'; 
   std::cout << b << '\n';
}