Add TupleDescGetDefault()

This unifies some repetitive code.

Note: I didn't push the "not found" error message into the new
function, even though all existing callers would be able to make use
of it.  Using the existing error handling as-is would probably require
exposing the Relation type via tupdesc.h, which doesn't seem
desirable.  (Or even if we changed it to just report the OID, it would
inject the concept of a relation containing the tuple descriptor into
tupdesc.h, which might be a layering violation.  Perhaps some further
improvements could be considered here separately.)

Discussion: https://www.postgresql.org/message-id/flat/52a125e4-ff9a-95f5-9f61-b87cf447e4da%40eisentraut.org

src/backend/access/common/tupdesc.c

@@ -927,3 +927,28 @@ BuildDescFromLists(const List *names, const List *types, const List *typmods, co
 
 	return desc;
 }
+
+/*
+ * Get default expression (or NULL if none) for the given attribute number.
+ */
+Node *
+TupleDescGetDefault(TupleDesc tupdesc, AttrNumber attnum)
+{
+	Node	   *result = NULL;
+
+	if (tupdesc->constr)
+	{
+		AttrDefault *attrdef = tupdesc->constr->defval;
+
+		for (int i = 0; i < tupdesc->constr->num_defval; i++)
+		{
+			if (attrdef[i].adnum == attnum)
+			{
+				result = stringToNode(attrdef[i].adbin);
+				break;
+			}
+		}
+	}
+
+	return result;
+}