Before this patch, the parser would execute:
- Select->expr_list.push_front()
- Select->expr_list.pop()
when parsing expressions lists, in the following rules:
- udf_expr_list
- expr_list
- ident_list

This is unnecessary, and introduces overhead due to the memory allocations
performed with Select->expr_list

With this patch, this code has been removed.
The list being parsed is maintained in the parser stack instead.

Also, 'udf_expr_list' has been renamed 'opt_udf_expr_list', since this
production can be empty.