WL#9167 Index merge hints.

Post-push fix:
Fix Clang build warnings about inconsistent usage of 'override'.

Author: jhauglid

sql/opt_hints.h

@@ -550,7 +550,7 @@ class Opt_hints_table : public Opt_hints
     @param thd   pointer to THD object
     @param str   pointer to String object
   */
-  virtual void append_name(THD *thd, String *str)
+  virtual void append_name(THD *thd, String *str) override
   {
     append_identifier(thd, str, get_name()->str, get_name()->length);
     get_parent()->append_name(thd, str);
@@ -562,7 +562,7 @@ class Opt_hints_table : public Opt_hints
     @param table      Pointer to TABLE_LIST object
   */
   void adjust_key_hints(TABLE_LIST *table);
-  virtual PT_hint *get_complex_hints(opt_hints_enum type);
+  virtual PT_hint *get_complex_hints(opt_hints_enum type) override;
 
   void set_resolved() override
   {

sql/parse_tree_hints.h

@@ -231,7 +231,7 @@ class PT_key_level_hint : public PT_hint
     @return  true in case of error,
              false otherwise
   */
-  virtual bool contextualize(Parse_context *pc);
+  virtual bool contextualize(Parse_context *pc) override;
   void append_args(THD *thd, String *str) const override;
 };