With this change, users are now able to run View AST command in
vscode within vscode workspaces that do not include the core libraries.
The relevant core library only needs to be installed in the package
cache.
Depending on the extraction order, before this change there might be multiple
`GlobalVariable`s per declared global variable. See the tests in
`cpp/ql/test/library-tests/variables/global`. This change ensures that only one
of those `GlobalVariable`s is visible to the user if we can locate a unique
definition. If not, the old situation persists.
Note that an exception needs to be made for templated variables. Here, the
definition refers to the non-instantiated template, while a declaration that
is not a definition refers to an instantiation. In case the instantiation refers
to a template parameter, the mangled names of the template and the instantiation
will be identical. This happens for example in the following case:
```
template <typename T>
T x = T(42); // Uninstantiated templated variable
template <typename T>
class C {
T y = x<T>; // Instantiation using a template parameter
};
```
Since the uninstantiated template and the instantiation are two different
entities, we do not unify them as described above.
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.
