We currently forward declare classes as we use them in functions.
But this breaks if classes with the same name were declared in the
parent namespace. As shown with this example
```C++
// Uncomment that line to make the code break
//struct SharedGlobals;
namespace ns {
// Is that a forward declaration in `ns`?
// Depends if it was declared before in the parent namespace
void foo(struct SharedGlobals *x) {}
// Actualy define ns::SharedGlobal
struct SharedGlobals { int x; };
int xyz() {
SharedGlobals globals;
foo(&globals);
}
}
```
So make sure we forward-declare the classes properly to be more robust
and be able to include generated file with namespace after a file
without namespace
CC #2909
