[BUILD] Fix reflection build for gcc-8

[tqchen]

https://discuss.tvm.ai/t/tvm-with-gcc-8-1-build-errors/286

The error was due to gcc-8 have some problem redirecting to the right operator T (where T is double, or int) inside the inside a template function. This PR introduces a workaround of that

Minimum Example to Demonstrate the Problem

#include <type_traits>

class MyClass {
 public:
  operator double() const {
    return 1;
  }
  template<typename T>
  operator T() const {
    static_assert(std::is_class<T>::value, "problem");
    return T();
  }
};

template<typename T>
void SetValue(const MyClass& obj, T* value) {
  *value = obj.operator T();
}

int main() {
  MyClass obj;
  // works fine                                                                                                                              
  obj.operator double();
  double x;
  // error;                                                                                                                                  
  SetValue(obj, &x);
}

[tqchen]

confirmed as a regression of gcc https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86246