A preparatory patch for MDEV-7284 INDEX: CREATE OR REPLACE.

Removing "bool Key::create_if_not_exists" and deriving Key from
DDL_options instead.

sql/sql_class.cc

@@ -112,13 +112,12 @@ bool Key_part_spec::operator==(const Key_part_spec& other) const
 */
 
 Key::Key(const Key &rhs, MEM_ROOT *mem_root)
-  :type(rhs.type),
+  :DDL_options(rhs),type(rhs.type),
   key_create_info(rhs.key_create_info),
   columns(rhs.columns, mem_root),
   name(rhs.name),
   option_list(rhs.option_list),
-  generated(rhs.generated),
-  create_if_not_exists(rhs.create_if_not_exists)
+  generated(rhs.generated)
 {
   list_copy_and_replace_each_value(columns, mem_root);
 }