When compiling GROUP BY Item_ref instances are dereferenced in 
setup_copy_fields(), i.e. replaced with the corresponding Item_field 
(if they point to one) or Item_copy_string for the other cases.
Since the Item_ref (in the Item_field case) is no longer used the information
about the aliases stored in it is lost.   
Fixed by preserving the column, table and DB alias on dereferencing Item_ref


mysql-test/r/metadata.result:
  Bug #20191: getTableName gives wrong or inconsistent result when using VIEWs
   - test case
mysql-test/t/metadata.test:
  Bug #20191: getTableName gives wrong or inconsistent result when using VIEWs
   - test case
sql/item.cc:
  Bug #20191: getTableName gives wrong or inconsistent result when using VIEWs
   - use the table and db name to fill up the metadata for columns
sql/sql_select.cc:
  Bug #20191: getTableName gives wrong or inconsistent result when using VIEWs
   - preserve the field, table and DB name on dereferencing an Item_ref