We usually don't change the name of structs between the struct name
itself and the name of the typedef. Additionally, structs that are
usually used via a typedef that hides being a pointer, are commonly
suffixed Data.  Change tupdesc code to follow those convention.

This is triggered by a future patch that intends to forward declare
TupleDescData in another header - keeping with the naming scheme makes
that easier to understand.

Author: Andres Freund
Discussion: https://postgr.es/m/20190114000701.y4ttcb74jpskkcfb@alap3.anarazel.de