[Clang][Sema] Diagnose unexpanded packs in the template argument lists of...

[Clang][Sema] Diagnose unexpanded packs in the template argument lists of function template specializations (#76677)

This diagnoses unexpanded packs in the _unqualified-id_ of a function
template specialization's _declarator-id_. For example:
```cpp
template<typename... Ts>
struct A
{
    template<typename U>
    void f();

    template<>
    void f<Ts>(); // error: explicit specialization contains unexpanded parameter pack 'Ts'
};
```

I moved the handling of template-id's so it happens right after we
determine whether we are declaring a function template/function template
specialization so diagnostics are issued in lexical order.