Cache TTRs in Model tools

include/revng/Model/ToolHelpers.h

@@ -213,8 +213,15 @@ public:
     return *Module;
   }
 
-  TupleTree<model::Binary> &getModel() { return Model; }
-  const TupleTree<model::Binary> &getModel() const { return Model; }
+  TupleTree<model::Binary> &getWriteableModel() {
+    Model.evictCachedReferences();
+    return Model;
+  }
+
+  const model::Binary &getReadOnlyModel() {
+    Model.cacheReferences();
+    return *std::as_const(Model);
+  }
 
 public:
   llvm::Error save(const llvm::Twine &Path, ModelOutputType::Values Type) {

tools/model/apply/Main.cpp

@@ -51,7 +51,7 @@ int main(int Argc, char *Argv[]) {
   if (not Diff)
     ExitOnError(Diff.takeError());
 
-  Diff->apply(Model->getModel());
+  Diff->apply(Model->getWriteableModel());
 
   auto DesiredOutput = Options.getDesiredOutput(Model->hasModule());
   ExitOnError(Model->save(Options.getPath(), DesiredOutput));

tools/model/export/type-graph/Main.cpp

@@ -43,7 +43,7 @@ int main(int Argc, char *Argv[]) {
     revng_abort(EC.message().c_str());
 
   TypeSystemPrinter TSPrinter(Out);
-  TSPrinter.print(*MaybeModel->getModel());
+  TSPrinter.print(MaybeModel->getReadOnlyModel());
 
   return EXIT_SUCCESS;
 }