[Clang][Sema] Correctly look up primary template for variable template specializations (#80359)

Consider the following:
```
namespace N0 {
  namespace N1 {
    template<typename T>
    int x1 = 0;
  }
  using namespace N1;
}
template<>
int N0::x1<int>;
```

According to [dcl.meaning.general] p3.3:
> - If the _declarator_ declares an explicit instantiation or a partial
or explicit specialization, the _declarator_ does not bind a name. If it
declares a class member, the terminal name of the _declarator-id_ is not
looked up; otherwise, **only those lookup results that are nominable in
`S` are considered when identifying any function template specialization
being declared**.

In particular, the requirement for lookup results to be nominal in the
lookup context of the terminal name of the _declarator-id_ only applies
to function template specializations -- not variable template
specializations. We currently reject the above declaration, but we do
(correctly) accept it if the using-directive is replaced with a `using`
declaration naming `N0::N1::x1`. This patch makes it so the above
specialization is (correctly) accepted.