This was already possible when the forward class declaration and the class
definition occurred in the same scope. However, there is a common C++ usage
pattern in which this is not the case (when only a pointer to the class is
needed). In this latter scenario we could not round trip between the (forward)
`DeclarationEntry` and the `Declaration`.
Effectively this changes the code to:
```
if exists(TypeDeclarationEntry e | e.getType() = this)
then result.getType() = this
else ...
```
We use `type_decls` instead to stay close to the original code.
