Pass along better debug info when accessing a non-grouped field in an aggregate expression

In particular, maintain the column name even after the conversion to an unnamed column reference.

Fixes #265 
maybe set `debug_info` to `"${}".format(position)` in this case? Looks like we'll lose the `$` otherwise.

In general, should we be using this everywhere we convert to `UnnamedAttributeRef`? I'm worried that at the end of the day our optimization rules might lead to names being used in places where the user didn't expect them.

(I know that this particular use is in the parser so the optimization thing isn't a problem.)