Partially flatten struct tupleDesc so that it can be used in DSM.

TupleDesc's attributes were already stored in contiguous memory after the
struct.  Go one step further and get rid of the array of pointers to
attributes so that they can be stored in shared memory mapped at different
addresses in each backend.  This won't work for TupleDescs with contraints
and defaults, since those point to other objects, but for many purposes
only attributes are needed.

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