Support multiple definitions

Sometimes declarations or usings have several definitions in the codebase. 

Currently, CodeCompass can not support multiple implementations in C++.

Ex1:
```cpp
namespace N {
  int a();
  int a(int);
}

using N::a; // ?
```

Ex2:
```cpp
template<class>  struct B; // ?
template<class>  struct B    {}; // impl1
template<>       struct B<A> {}; // impl2
```