[Clang][Sema] Fix crash when using name of UnresolvedUsingValueDecl with...

[Clang][Sema] Fix crash when using name of UnresolvedUsingValueDecl with template arguments (#83842)

The following snippet causes a crash:
```
template<typename T>
struct A : T {
  using T::f;
  void f();

  void g() {
    f<int>(); // crash here
  }
};
```
This happens because we cast the result of `getAsTemplateNameDecl` as a
`TemplateDecl` in `Sema::ClassifyName`, which we cannot do for an
`UnresolvedUsingValueDecl`. This patch fixes the crash by considering a name
to be that of a template if _any_ function declaration is found per [temp.names] p3.3.