Add name method to builtin functions

This is useful to make the dot output more informative.

src/solvers/refinement/string_refinement_util.cpp

@@ -631,7 +631,7 @@ void string_dependenciest::output_dot(std::ostream &stream) const
   const auto node_to_string = [&](const nodet &n) { // NOLINT
     std::stringstream ostream;
     if(n.kind == nodet::BUILTIN)
-      ostream << "builtin_" << n.index;
+      ostream << builtin_function_nodes[n.index]->name() << n.index;
     else
       ostream << '"' << format(string_nodes[n.index].expr) << '"';
     return ostream.str();

src/solvers/refinement/string_refinement_util.h

@@ -161,6 +161,8 @@ class string_builtin_functiont
   virtual optionalt<exprt>
   eval(const std::function<exprt(const exprt &)> &get_value) const = 0;
 
+  virtual std::string name() const = 0;
+
 protected:
   string_builtin_functiont() = default;
 };
@@ -212,6 +214,11 @@ class string_concat_char_builtin_functiont
   std::vector<mp_integer> eval(
     const std::vector<mp_integer> &input_value,
     const std::vector<mp_integer> &args_value) const override;
+
+  std::string name() const override
+  {
+    return "concat_char";
+  }
 };
 
 /// String inserting a string into another one
@@ -247,6 +254,11 @@ class string_insertion_builtin_functiont : public string_builtin_functiont
   optionalt<exprt>
   eval(const std::function<exprt(const exprt &)> &get_value) const override;
 
+  std::string name() const override
+  {
+    return "insert";
+  }
+
 protected:
   string_insertion_builtin_functiont() = default;
 };
@@ -263,6 +275,11 @@ class string_concatenation_builtin_functiont final
     const std::vector<mp_integer> &input1_value,
     const std::vector<mp_integer> &input2_value,
     const std::vector<mp_integer> &args_value) const override;
+
+  std::string name() const override
+  {
+    return "concat";
+  }
 };
 
 /// String creation from other types