optimizer does not honor IGNORE INDEX
 - Allow an index to be used for sorting the table 
   instead of filesort only if it is not disabled by
   IGNORE INDEX.


mysql-test/r/group_by.result:
  Bug #21174: Index degrades sort performance and 
               optimizer does not honor IGNORE INDEX
   - test case
mysql-test/t/group_by.test:
  Bug #21174: Index degrades sort performance and 
               optimizer does not honor IGNORE INDEX
   - test case