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

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Sirraide wrote:

So after talking over some of the more horrible edge cases w/ Corentin for a while, it looks like implementing this requires supporting unexpanded packs in nested name specifiers, which we don’t, but he has a patch for that, so I’ll be taking a look at that next.

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