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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> Still need to add tests for [CWG 2917](https://cplusplus.github.io/CWG/issues/2917.html), which I filed while working on this, to the DR test suite proper. (@Endilll Sorry for asking this again, but do I need to run the script that updates the DR tests as part of this, or should that be done in a separate pr before this?)

If you do this now, you'll get a lot of non-trivial updates. Ideally you should wait for #97200 (@MitalAshok any blockers there?).

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