USR generation for anonymous namespaces

The clang USR generator will generate the same USR for anonymous namespaces, regardless of what the enclosing namespace is. For example:
```cpp
namespace 
{ 
    void f(); // #1
}

namespace N 
{
    namespace 
    {
        void f(); // #2
    }
};
```
Clang (incorrectly) generates the same USR for `#1` and `#2`.
