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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

@Sirraide CWG looked at 2917 last week, and changed the status of the issue. Can you revisit your test for it? 
https://cplusplus.github.io/CWG/issues/2917.html

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