Validity checks for nested set functions
were not taking into account that the enclosed
set function may be on a nest level that is
lower than the nest level of the enclosing set
function.
Fixed by :
 - propagating max_sum_func_level
up the enclosing set functions chain.
 - updating the max_sum_func_level of the 
   enclosing set function when the enclosed set
   function is aggregated above or on the same
   nest level of as the level of the enclosing 
   set function.
 - updating the max_arg_level of the enclosing
   set function on a reference that refers to
   an item above or on the same nest level
   as the level of the enclosing set function.
 - Treating both Item_field and Item_ref as possibly
   referencing items from outer nest levels.


mysql-test/r/subselect.result:
  Bug #27363: test cases
mysql-test/t/subselect.test:
  Bug #27363: test cases
sql/item.cc:
  Bug #27363:
  Treat the reference as an outer reference for the
  enclosing set function even if it's referencing
  an item that is above the nest level of the
  enclosing set function.
  Consider both Item_field and Item_ref.
sql/item_sum.cc:
  Bug #27363: Use the enclosed set function aggregation
  level to mark the enclosing set function even 
  if it's aggregated on a level that is above the 
  nest level of the enclosing set function.
  Pass max_sum_func_level up the accending branch of the
  recursion because it must take into account each
  directly or indirectly nested set function.