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

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Sirraide wrote:

Ok, I think I’ve addressed everything now. I’ve also added the feature-test macro back since we won’t be splitting this into multiple prs after all (there are still friend declarations that we don’t handle properly, but that encompasses all friend declarations w/ a dependent NNS irrespective of whether they’re pack expansions or not, so that’s a separate problem).

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