[clang] [clang] Fix the crash when dumping deserialized decls (PR #133395)

[Ilya Biryukov via cfe-commits]

================
@@ -103,15 +120,30 @@ class DeserializedDeclsDumper : public DelegatingDeserializationListener {
       : DelegatingDeserializationListener(Previous, DeletePrevious) {}
 
   void DeclRead(GlobalDeclID ID, const Decl *D) override {
-    llvm::outs() << "PCH DECL: " << D->getDeclKindName();
-    if (const NamedDecl *ND = dyn_cast<NamedDecl>(D)) {
-      llvm::outs() << " - ";
-      ND->printQualifiedName(llvm::outs());
+    PendingDecls.push_back(D);
+    DelegatingDeserializationListener::DeclRead(ID, D);
+  }
+  void FinishedDeserializing() override {
+    auto Decls = std::move(PendingDecls);
+    for (const auto *D : Decls) {
+      llvm::outs() << "PCH DECL: " << D->getDeclKindName();
+      if (const NamedDecl *ND = dyn_cast<NamedDecl>(D)) {
+        llvm::outs() << " - ";
+        ND->printQualifiedName(llvm::outs());
+      }
+      llvm::outs() << "\n";
     }
-    llvm::outs() << "\n";
 
-    DelegatingDeserializationListener::DeclRead(ID, D);
+    if (!PendingDecls.empty()) {
----------------
ilya-biryukov wrote:

Our theory is that `printQualifiedName` can start deserializing more.

At the point where the callback is called now, we have already updated the state of `ASTReader` such that **more deserialization will cause this callback to be called again**. I.e. this should be a possible stack trace:
```
OurListener::FinishDeserializing()
ASTReader::FinishDeserializing()
ASTReader::doSomeDeserialization()
...
Decl::printQualifiedName()
OurListener::FinishDeserializing()
ASTReader::FinishDeserializing()
```

We probably need to figure out an API that does not require handling situations like this.
E.g. right now it means (I believe) that this condition will never be true: the recursive serialization will just remember all other decls and print their names and clear the `PendingDecls` vector.

https://github.com/llvm/llvm-project/pull/133395