Make naming of tupdesc related structs more consistent with the rest of PG.

Make naming of tupdesc related structs more consistent with the rest of PG.

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