The server failed on EXPLAIN of some GROUP_CONCAT() expressions.

The fix consists of two parts.

The 1st one (item_sum.{h,cc}) simplifies the code base by the removal of
superfluous Item_sum::orig_args and Item_sum::tmp_orig_args fields.

The 2nd one (item.{h,cc}) improves Item_field::print() to output column
names with 3D-qualified identifiers:
<original database>.<original table>.<column name> instead of 1D identifiers
(<column name>).