[libclang] Inherited constructors not reflected on CXType

In this example:

```c++
class A {
public:
A(int);
A(const char*);
};

class B : public A {
public:
using A::A;
};
```

when I use `clang_visitCXXMethods` on the `CXType` for `B`, I get a default constructor, a copy constructor, and a move constructor — exactly as if the `using` declaration were not present.

I can walk the decl for B and figure out what's going on, but it's a pain. It would be much easier if the `CXType` had the correct constructor set.