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

[via cfe-commits]

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@@ -1087,6 +1087,7 @@ void JSONNodeDumper::VisitAccessSpecDecl(const AccessSpecDecl *ASD) {
 void JSONNodeDumper::VisitFriendDecl(const FriendDecl *FD) {
   if (const TypeSourceInfo *T = FD->getFriendType())
     JOS.attribute("type", createQualType(T->getType()));
+  attributeOnlyIfTrue("variadic", FD->isVariadic());
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cor3ntin wrote:

```suggestion
  attributeOnlyIfTrue("isPackExpansion", FD->isVariadic());
```

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