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

[via cfe-commits]

Sirraide wrote:

> I think it would be fine and probably preferable to do that feature in stages / multiple PRS

That makes sense to me, yeah. So the next step would be supporting dependent friend declarations properly, I take it? I.e. cases such as
```c++
template <typename T>
friend class Foo<T>::Nested;
```

And yeah, in that case, I’ll remove the release note etc. for now.

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