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

[via cfe-commits]

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@@ -2694,6 +2694,8 @@ void TextNodeDumper::VisitAccessSpecDecl(const AccessSpecDecl *D) {
 void TextNodeDumper::VisitFriendDecl(const FriendDecl *D) {
   if (TypeSourceInfo *T = D->getFriendType())
     dumpType(T->getType());
+  if (D->isVariadic())
+    OS << " variadic";
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cor3ntin wrote:

```suggestion
    OS << " ...";
```

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