Fix ruleutils issues with dropped cols in functions-returning-composite.

Due to lack of concern for the case in the dependency code, it's
possible to drop a column of a composite type even though stored
queries have references to the dropped column via functions-in-FROM
that return the composite type.  There are "soft" references,
namely FROM-clause aliases for such columns, and "hard" references,
that is actual Vars referring to them.  The right fix for hard
references is to add dependencies preventing the drop; something
we've known for many years and not done (and this commit still doesn't
address it).  A "soft" reference shouldn't prevent a drop though.
We've been around on this before (cf. 9b35ddce9, 2c4debbd0), but
nobody had noticed that the current behavior can result in dump/reload
failures, because ruleutils.c can print more column aliases than the
underlying composite type now has.  So we need to rejigger the
column-alias-handling code to treat such columns as dropped and not
print aliases for them.

Rather than writing new code for this, I used expandRTE() which already
knows how to figure out which function result columns are dropped.
I'd initially thought maybe we could use expandRTE() in all cases, but
that fails for EXPLAIN's purposes, because the planner strips a lot of
RTE infrastructure that expandRTE() needs.  So this patch just uses it
for unplanned function RTEs and otherwise does things the old way.

If there is a hard reference (Var), then removing the column alias
causes us to fail to print the Var, since there's no longer a name
to print.  Failing seems less desirable than printing a made-up
name, so I made it print "?dropped?column?" instead.

Per report from Timo Stolz.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/5c91267e-3b6d-5795-189c-d15a55d61dbb@nullachtvierzehn.de

src/test/regress/expected/create_view.out

@@ -1546,17 +1546,26 @@ select * from tt14v;
 begin;
 -- this perhaps should be rejected, but it isn't:
 alter table tt14t drop column f3;
--- f3 is still in the view ...
+-- column f3 is still in the view, sort of ...
 select pg_get_viewdef('tt14v', true);
-         pg_get_viewdef         
---------------------------------
-  SELECT t.f1,                 +
-     t.f3,                     +
-     t.f4                      +
-    FROM tt14f() t(f1, f3, f4);
+         pg_get_viewdef          
+---------------------------------
+  SELECT t.f1,                  +
+     t."?dropped?column?" AS f3,+
+     t.f4                       +
+    FROM tt14f() t(f1, f4);
 (1 row)
 
--- but will fail at execution
+-- ... and you can even EXPLAIN it ...
+explain (verbose, costs off) select * from tt14v;
+               QUERY PLAN               
+----------------------------------------
+ Function Scan on testviewschm2.tt14f t
+   Output: t.f1, t.f3, t.f4
+   Function Call: tt14f()
+(3 rows)
+
+-- but it will fail at execution
 select f1, f4 from tt14v;
  f1  | f4 
 -----+----

src/test/regress/sql/create_view.sql

@@ -526,9 +526,11 @@ begin;
 -- this perhaps should be rejected, but it isn't:
 alter table tt14t drop column f3;
 
--- f3 is still in the view ...
+-- column f3 is still in the view, sort of ...
 select pg_get_viewdef('tt14v', true);
--- but will fail at execution
+-- ... and you can even EXPLAIN it ...
+explain (verbose, costs off) select * from tt14v;
+-- but it will fail at execution
 select f1, f4 from tt14v;
 select * from tt14v;