[clang][ASTImporter] Fix possible crash "given incorrect InsertPos for specialization". (#89887)

In some situations a new `VarTemplateSpecializationDecl` (for the same
template) can be added during import of another one. The "insert
position" that is used to insert the current object into the list of
specializations is stored at start of the import and is used later. If
the list changes before the insertion the position is not valid any
more.

Author: balazske

clang/lib/AST/ASTImporter.cpp

@@ -6565,6 +6565,11 @@ ExpectedDecl ASTNodeImporter::VisitVarTemplateSpecializationDecl(
       return D2;
   }
 
+  // Update InsertPos, because preceding import calls may have invalidated
+  // it by adding new specializations.
+  if (!VarTemplate->findSpecialization(TemplateArgs, InsertPos))
+    VarTemplate->AddSpecialization(D2, InsertPos);
+
   QualType T;
   if (Error Err = importInto(T, D->getType()))
     return std::move(Err);
@@ -6603,8 +6608,6 @@ ExpectedDecl ASTNodeImporter::VisitVarTemplateSpecializationDecl(
   if (FoundSpecialization)
     D2->setPreviousDecl(FoundSpecialization->getMostRecentDecl());
 
-  VarTemplate->AddSpecialization(D2, InsertPos);
-
   addDeclToContexts(D, D2);
 
   // Import the rest of the chain. I.e. import all subsequent declarations.