[clang] [Clang] Implement C++26’s P2893R3 ‘Variadic friends’ (PR #101448)

[via cfe-commits]

cor3ntin wrote:

> Is there ever a situation that would cause us to have to instantiate a FriendPackDecl in the TemplateDeclInstantiator? We do that for UsingPackDecls, but those are sufficiently different enough from the former that I’m not sure about it. I at least haven’t been able to come up w/ any cases that would cause us to try and instantiate one.

That would be needed for the 

```cpp
template <typename Ts>
struct S {
  friend class S<Ts>::Nested...;
};
```

case, I think?


https://github.com/llvm/llvm-project/pull/101448