add_to_front method for code_with_references_listt

This will make it easier to add declarations at the begginning of a list of
codet after the rest of the code has been generated.

jbmc/src/java_bytecode/code_with_references.cpp

@@ -68,3 +68,9 @@ void code_with_references_listt::append(code_with_references_listt &&other)
 {
   list.splice(list.end(), other.list);
 }
+
+void code_with_references_listt::add_to_front(code_without_referencest code)
+{
+  auto ptr = std::make_shared<code_without_referencest>(std::move(code));
+  list.emplace_front(std::move(ptr));
+}

jbmc/src/java_bytecode/code_with_references.h

@@ -101,6 +101,8 @@ public:
   void add(reference_allocationt ref);
 
   void append(code_with_references_listt &&other);
+
+  void add_to_front(code_without_referencest code);
 };
 
 #endif // CPROVER_JAVA_BYTECODE_CODE_WITH_REFERENCES_H