Fix toast table creation.

Instead of using slightly-too-clever heuristics to decide when we must
create a TOAST table, just check whether one is needed every time the
table is altered.  Checking whether a toast table is needed is cheap
enough that we needn't worry about doing it on every ALTER TABLE command,
and the previous coding is apparently prone to accidental breakage:
commit 04e17bae50 broken ALTER TABLE ..
SET STORAGE, which moved some actions from AT_PASS_COL_ATTRS to
AT_PASS_MISC, and commit 6c57239985 broke
ALTER TABLE .. ADD COLUMN by changing the way that adding columns
recurses into child tables.

Noah Misch, with one comment change by me

src/test/regress/sql/alter_table.sql

@@ -1133,6 +1133,16 @@ alter table recur1 add column f2 recur2; -- fails
 alter table recur1 add column f2 int;
 alter table recur1 alter column f2 type recur2; -- fails
 
+-- SET STORAGE may need to add a TOAST table
+create table test_storage (a text);
+alter table test_storage alter a set storage plain;
+alter table test_storage add b int default 0; -- rewrite table to remove its TOAST table
+alter table test_storage alter a set storage extended; -- re-add TOAST table
+
+select reltoastrelid <> 0 as has_toast_table
+from pg_class
+where oid = 'test_storage'::regclass;
+
 --
 -- lock levels
 --