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

[via cfe-commits]

https://github.com/cor3ntin commented:

Looks generally good from a cursory review.
I think we want to add tests for modules / in the test/PCH directory.

We probably want that as an extension indeed, no reason not to

We probably want a separate PR to add the pack indexing macro. I did not add it initially because i was concerned the implementation was not mature enough, but i think we fond most of the bugs.

I think it's fine to set the feature macro for that now with the assumption that we will find all the bugs in the next 6 months.

There seem to be some missing components, like the json node printer.

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