[WIP] [core] Force compilation error on variable shadow

[dentiny]

The following code currently compiles.

class Status {
 template <typename... T>
  Status &operator<<(T &&...msg) {
    const std::string shadowed_variable = "hahaha";  // variable shadowing here
    return *this;
  }

std::string shadowed_variable; // data member here
};

It's really error-prune and we should definitely ban it.