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

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Sirraide wrote:

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

We already have tests for PCHs, but I can add some more for modules.

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

We allow `friend` type declarations in all language modes, so I’ll do that too.

> 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.

I’ll open an NFC pr for that.

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

I always forget about something when I add an AST node...

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