Added function last_value() which returns the last value but evalutes all arguments as a side effect.
Original patch by Eric Herman

client/mysql.cc:
  Added LAST_VALUE
mysql-test/r/last_value.result:
  Testing of LAST_VALUE
mysql-test/t/last_value.test:
  Testing of LAST_VALUE
sql/item_func.cc:
  Added LAST_VALUE()
sql/item_func.h:
  Added LAST_VALUE()
sql/lex.h:
  Added LAST_VALUE()
sql/sql_yacc.yy:
  Added LAST_VALUE()