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

[via cfe-commits]

cor3ntin 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.

I think it would be fine and probably preferable to do that feature in stages / multiple PRS

1. This
2. Nested dependent non-pack friends
3. Packs

This is big enough to review 
(If we go that way we can remove the changelog/cxx_status/feature test macro for now)


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