MWL#17: Table elimination

- Make elimination work with aggregate functions. The problem was that aggregate functions 
  reported all table bits in used_tables(), and that prevented table elimination. Fixed by 
  making aggregate functions return more correct value from used_tables(). 

mysql-test/r/ps_11bugs.result:
  MWL#17: Table elimination
  - Update test results. The difference is because of Item_ref change: outer references to constants
    are now recognized as constants, too.
mysql-test/r/subselect.result:
  - Update test results. The difference is because of Item_ref change: outer references to constants
    are now recognized as constants, too.
mysql-test/r/table_elim.result:
  MWL#17: Table elimination
  - Check that elimination works in presense of aggreagate functions
mysql-test/t/table_elim.test:
  MWL#17: Table elimination
  - Check that elimination works in presense of aggreagate functions
sql/item.h:
  MWL#17: Table elimination
  - Add Item_ref::const_item() which calls (*ref)->const_item(). Before this diff Item_ref used the 
    default implementation of const_item(){ return used_tables()==0; }. This is no longer true, as 
    COUNT(*) now has used_tables()==0 but const_item()==FALSE.
sql/item_sum.cc:
  MWL#17: Table elimination
  - Make Item_sum() and it descendants not to return all bits in used_tables(). This is needed 
    because otherwise table elimination can't work in presense of aggregate functions
  - COUNT(*) now has used_tables()==0 and const_item()==FALSE. Had to change 
    Item_ref::const_item() to account for this.
sql/item_sum.h:
  MWL#17: Table elimination
  - Add comments

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