WL#5370 Keep forward-compatibility when changing

'CREATE TABLE IF NOT EXISTS ... SELECT' behaviour
BUG#47132, BUG#47442, BUG49494, BUG#23992 and BUG#48814 will disappear
automatically after the this patch.
BUG#55617 is fixed by this patch too.
            
This is the 5.5 part.
It implements:
- 'CREATE TABLE IF NOT EXISTS ... SELECT' statement will not insert
  anything and binlog anything if the table already exists.
  It only generate a warning that table already exists.
- A couple of test cases for the behavior changing.

sql/sql_base.cc

@@ -2540,10 +2540,6 @@ open_table_get_mdl_lock(THD *thd, Open_table_context *ot_ctx,
     is never opened. In both cases, metadata locks are always taken according
     to the lock strategy.
 
-    If the lock strategy is OTLS_DOWNGRADE_IF_EXISTS and opening the table
-    is successful, the exclusive metadata lock acquired by the caller
-    is downgraded to a shared lock.
-
   RETURN
     TRUE  Open failed. "action" parameter may contain type of action
           needed to remedy problem before retrying again.
@@ -2952,15 +2948,6 @@ bool open_table(THD *thd, TABLE_LIST *table_list, MEM_ROOT *mem_root,
 
   mysql_mutex_unlock(&LOCK_open);
 
-  /*
-    In CREATE TABLE .. If NOT EXISTS .. SELECT we have found that
-    table exists now we should downgrade our exclusive metadata
-    lock on this table to SW metadata lock.
-  */
-  if (table_list->lock_strategy == TABLE_LIST::OTLS_DOWNGRADE_IF_EXISTS &&
-      !(flags & MYSQL_OPEN_HAS_MDL_LOCK))
-    mdl_ticket->downgrade_exclusive_lock(MDL_SHARED_WRITE);
-
   table->mdl_ticket= mdl_ticket;
 
   table->next= thd->open_tables;		/* Link into simple list */