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

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Sirraide wrote:

> we need to update the test and the status for bookkeeping purposes

In that case, I’ll leave the implementation of this as is for now and just add a FIXME and update the dr status.

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