Use override in all the places where it is possible

This will avoid -Winconsistent-overrides in the future. Done using the
-Wsuggest-override warning of GCC 5.1

BUG=

Change-Id: I707a649dc368f5dd1e139fd144370abcac0b6263
Reviewed-on: https://chromium-review.googlesource.com/294920
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/compiler/preprocessor/MacroExpander.h b/src/compiler/preprocessor/MacroExpander.h
index 5a0c775..aa7b783 100644
--- a/src/compiler/preprocessor/MacroExpander.h
+++ b/src/compiler/preprocessor/MacroExpander.h
@@ -24,9 +24,9 @@
 {
   public:
     MacroExpander(Lexer *lexer, MacroSet *macroSet, Diagnostics *diagnostics);
-    virtual ~MacroExpander();
+    ~MacroExpander() override;
 
-    virtual void lex(Token *token);
+    void lex(Token *token) override;
 
   private:
     PP_DISALLOW_COPY_AND_ASSIGN(MacroExpander);