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

[via cfe-commits]

Sirraide wrote:

> we are implementing the proposed resolution, which is the second set of change in that issue. we should _not_ implement the "suggested resolution" as discussions are still ongoing (notably concerns of it being too restrictive).

Well, the suggested resolution, i.e. *accepting*
```c++
template<class... Ts>
struct VS {
    template<class U>
    friend class C<Ts>::Nested...; // now ill-formed
};
 ```

is what’s already implemented. The proposed resolution rejects this instead.


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