Add QT_ITEM_IDENT_DISABLE_DB_TABLE_NAMES flag for Item::print

It allows to skip db and table name when printing table column references.

sql/item.cc

@@ -3193,6 +3193,12 @@ void Item_ident::print(String *str, enum_query_type query_type)
       use_db_name= use_table_name= false;
   }
 
+  if ((query_type & QT_ITEM_IDENT_DISABLE_DB_TABLE_NAMES))
+  {
+    // Don't print db or table name irrespective of any other settings.
+    use_db_name= use_table_name= false;
+  }
+
   if (!field_name.str || !field_name.str[0])
   {
     append_identifier(thd, str, STRING_WITH_LEN("tmp_field"));

sql/mysqld.h

@@ -871,6 +871,12 @@ enum enum_query_type
                         QT_ITEM_SUBSELECT_ID_ONLY,
 
   QT_SHOW_SELECT_NUMBER= (1<<10),
+
+  /// Do not print database name or table name in the identifiers (even if
+  /// this means the printout will be ambigous). It is assumed that the caller
+  ///  passing this flag knows what they are doing.
+  QT_ITEM_IDENT_DISABLE_DB_TABLE_NAMES= (1 <<11),
+
   /// This is used for EXPLAIN EXTENDED extra warnings / Be more detailed
   /// Be more detailed than QT_EXPLAIN.
   /// Perhaps we should eventually include QT_ITEM_IDENT_SKIP_CURRENT_DATABASE