[include-cleaner] Report all specializations if the primary template is introduced by a using-decl.

This will fix unused-include false positive.

```
// primary.h
namespace ns {
template<class T1, class T2> class Z {}; // primary template
}

// partial.h
namespace ns {
template<class T> class Z<T, T*> {};     // partial specialization
}

// main.cpp

using ns::Z; // refs to the primary
void k() {
  Z<int, int*> z; // use the partial specialization
}
```

Differential Revision: https://reviews.llvm.org/D152345