Adding missing overrides

Clang warns for files that inconsistently use the override keyword so
added the missing ones to jsil_language.

Author: thk123

src/jsil/jsil_language.h

@@ -25,49 +25,49 @@ class jsil_languaget:public languaget
   virtual bool preprocess(
     std::istream &instream,
     const std::string &path,
-    std::ostream &outstream);
+    std::ostream &outstream) override;
 
   virtual bool parse(
     std::istream &instream,
-    const std::string &path);
+    const std::string &path) override;
 
   virtual bool typecheck(
     symbol_tablet &context,
-    const std::string &module);
+    const std::string &module) override;
 
-  virtual bool final(symbol_tablet &context);
+  virtual bool final(symbol_tablet &context) override;
 
-  virtual void show_parse(std::ostream &out);
+  virtual void show_parse(std::ostream &out) override;
 
   virtual ~jsil_languaget();
   jsil_languaget() { }
 
   virtual bool from_expr(
     const exprt &expr,
     std::string &code,
-    const namespacet &ns);
+    const namespacet &ns) override;
 
   virtual bool from_type(
     const typet &type,
     std::string &code,
-    const namespacet &ns);
+    const namespacet &ns) override;
 
   virtual bool to_expr(
     const std::string &code,
     const std::string &module,
     exprt &expr,
-    const namespacet &ns);
+    const namespacet &ns) override;
 
-  virtual std::unique_ptr<languaget> new_language()
+  virtual std::unique_ptr<languaget> new_language() override
   { return util_make_unique<jsil_languaget>(); }
 
-  virtual std::string id() const { return "jsil"; }
-  virtual std::string description() const
+  virtual std::string id() const override { return "jsil"; }
+  virtual std::string description() const override
   { return "Javascript Intermediate Language"; }
-  virtual std::set<std::string> extensions() const;
+  virtual std::set<std::string> extensions() const override;
 
-  virtual void modules_provided(std::set<std::string> &modules);
-  virtual bool interfaces(symbol_tablet &symbol_table);
+  virtual void modules_provided(std::set<std::string> &modules) override;
+  virtual bool interfaces(symbol_tablet &symbol_table) override;
 
   virtual bool generate_start_function(
     const class symbolt &entry_function_symbol,