Re: BUG #17439: DROP FUNCTION functionName(); drops associated generated column without using CASCADE

I wrote:
> Ugh ... this opens a much larger can of worms than I thought.

After some fooling around, here's a draft patch for this.

I needed functions to convert between pg_attrdef OIDs and owning
column's table OID + attnum. There was already some ad-hoc code
for that in objectaddress.c, which I extracted into standalone
functions. It seemed cleaner to put those into heap.c (beside
StoreAttrDefault) than keep them in objectaddress.c; but perhaps
someone else will see that differently. I'm about half tempted
to shove StoreAttrDefault, RemoveAttrDefault, and these new
functions into a new file catalog/pg_attrdef.c, just to make heap.c
a bit smaller. But I didn't undertake that here.

Otherwise it seems mostly straightforward, but I remain concerned
that I've missed place(s) that depend on the previous arrangement.

regards, tom lane