crashes server

When creating temporary table that contains aggregate functions a 
non-reversible source transformation was performed to redirect aggregate
function arguments towards temporary table columns.
This caused EXPLAIN EXTENDED to fail because it was trying to resolve
references to the (freed) temporary table.
Fixed by preserving the original aggregate function arguments and
using them (instead of the transformed ones) for EXPLAIN EXTENDED.

mysql-test/r/explain.result:
  Bug#34773: test case
mysql-test/t/explain.test:
  Bug#34773: test case
sql/item.cc:
  Bug#34773: use accessor functions instead of public members
sql/item_sum.cc:
  Bug#34773: 
   - Encapsulate the arguments into Item_sum and
     provide accessor and mutator methods 
   - print the orginal arguments (if present)
     in EXPLAIN EXTENDED
   - preserve the original arguments list.
sql/item_sum.h:
  Bug#34773: 
   - Encapsulate the arguments into Item_sum and
     provide accessor and mutator methods 
   - print the orginal arguments (if present)
     in EXPLAIN EXTENDED
   - preserve the original arguments list.
sql/opt_range.cc:
  Bug#34773: use accessor functions instead of public members
sql/opt_sum.cc:
  Bug#34773: use accessor functions instead of public members
sql/sql_select.cc:
  Bug#34773: use accessor functions instead of public members