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

[via cfe-commits]

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@@ -177,6 +190,8 @@ class FriendDecl final
     UnsupportedFriend = Unsupported;
   }
 
+  bool isVariadic() const { return EllipsisLoc.isValid(); }
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cor3ntin wrote:

```suggestion
  bool isPackExpansion() const { return EllipsisLoc.isValid(); }
```

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