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/translator/SymbolTable.h b/src/compiler/translator/SymbolTable.h
index ba578d7..66b9174 100644
--- a/src/compiler/translator/SymbolTable.h
+++ b/src/compiler/translator/SymbolTable.h
@@ -109,13 +109,8 @@
           unionArray(0)
     {
     }
-    virtual ~TVariable()
-    {
-    }
-    virtual bool isVariable() const
-    {
-        return true;
-    }
+    ~TVariable() override {}
+    bool isVariable() const override { return true; }
     TType &getType()
     {
         return type;
@@ -229,11 +224,8 @@
     {
         relateToExtension(ext);
     }
-    virtual ~TFunction();
-    virtual bool isFunction() const
-    {
-        return true;
-    }
+    ~TFunction() override;
+    bool isFunction() const override { return true; }
 
     static TString mangleName(const TString &name)
     {
@@ -250,7 +242,7 @@
         mangledName = nullptr;
     }
 
-    const TString &getMangledName() const
+    const TString &getMangledName() const override
     {
         if (mangledName == nullptr)
         {