Change tupledesc->attrs[n] to TupleDescAttr(tupledesc, n).

This is a mechanical change in preparation for a later commit that
will change the layout of TupleDesc.  Introducing a macro to abstract
the details of where attributes are stored will allow us to change
that in separate step and revise it in future.

Author: Thomas Munro, editorialized by Andres Freund
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAEepm=0ZtQ-SpsgCyzzYpsXS6e=kZWqk3g5Ygn3MDV7A8dabUA@mail.gmail.com