Ensure that whole-row junk Vars are always of composite type.

The EvalPlanQual machinery assumes that whole-row Vars generated for the
outputs of non-table RTEs will be of composite types.  However, for the
case where the RTE is a function call returning a scalar type, we were
doing the wrong thing, as a result of sharing code with a parser case
where the function's scalar output is wanted.  (Or at least, that's what
that case has done historically; it does seem a bit inconsistent.)

To fix, extend makeWholeRowVar's API so that it can support both use-cases.
This fixes Belinda Cussen's report of crashes during concurrent execution
of UPDATEs involving joins to the result of UNNEST() --- in READ COMMITTED
mode, we'd run the EvalPlanQual machinery after a conflicting row update
commits, and it was expecting to get a HeapTuple not a scalar datum from
the "wholerowN" variable referencing the function RTE.

Back-patch to 9.0 where the current EvalPlanQual implementation appeared.

In 9.1 and up, this patch also fixes failure to attach the correct
collation to the Var generated for a scalar-result case.  An example:
regression=# select upper(x.*) from textcat('ab', 'cd') x;
ERROR:  could not determine which collation to use for upper() function

src/backend/optimizer/prep/preptlist.c

@@ -129,7 +129,8 @@ preprocess_targetlist(PlannerInfo *root, List *tlist)
 			/* Not a table, so we need the whole row as a junk var */
 			var = makeWholeRowVar(rt_fetch(rc->rti, range_table),
 								  rc->rti,
-								  0);
+								  0,
+								  false);
 			snprintf(resname, sizeof(resname), "wholerow%u", rc->rowmarkId);
 			tle = makeTargetEntry((Expr *) var,
 								  list_length(tlist) + 1,