Solve cross-version-upgrade testing problem induced by 1fb57af92.

Renaming varchar_transform to varchar_support had a side effect
I hadn't foreseen: the core regression tests leave around a
transform object that relies on that function, so the name
change breaks cross-version upgrade tests, because the name
used in the older branches doesn't match.

Since the dependency on varchar_transform was chosen with the
aid of a dartboard anyway (it would surely not work as a
language transform support function), fix by just choosing
a different random builtin function with the right signature.
Also add some comments explaining why this isn't horribly unsafe.

I chose to make the same substitution in a couple of other
copied-and-pasted test cases, for consistency, though those
aren't directly contributing to the testing problem.

Per buildfarm.  Back-patch, else it doesn't fix the problem.

src/bin/pg_dump/t/002_pg_dump.pl

@@ -3922,9 +3922,9 @@ qr/^\QCREATE DEFAULT CONVERSION dump_test.test_conversion FOR 'LATIN1' TO 'UTF8'
 		all_runs     => 1,
 		create_order => 34,
 		create_sql =>
-'CREATE TRANSFORM FOR int LANGUAGE SQL (FROM SQL WITH FUNCTION varchar_transform(internal), TO SQL WITH FUNCTION int4recv(internal));',
+'CREATE TRANSFORM FOR int LANGUAGE SQL (FROM SQL WITH FUNCTION prsd_lextype(internal), TO SQL WITH FUNCTION int4recv(internal));',
 		regexp =>
-qr/CREATE TRANSFORM FOR integer LANGUAGE sql \(FROM SQL WITH FUNCTION pg_catalog\.varchar_transform\(internal\), TO SQL WITH FUNCTION pg_catalog\.int4recv\(internal\)\);/m,
+qr/CREATE TRANSFORM FOR integer LANGUAGE sql \(FROM SQL WITH FUNCTION pg_catalog\.prsd_lextype\(internal\), TO SQL WITH FUNCTION pg_catalog\.int4recv\(internal\)\);/m,
 		like => {
 			binary_upgrade           => 1,
 			clean                    => 1,

src/test/modules/test_ddl_deparse/expected/create_transform.out

@@ -5,9 +5,11 @@
 -- The function FROM SQL should have internal as single argument as well
 -- as return type. The function TO SQL should have as single argument
 -- internal and as return argument the datatype of the transform done.
--- pl/plpgsql does not authorize the use of internal as data type.
+-- We choose some random built-in functions that have the right signature.
+-- This won't actually be used, because the SQL function language
+-- doesn't implement transforms (there would be no point).
 CREATE TRANSFORM FOR int LANGUAGE SQL (
-    FROM SQL WITH FUNCTION varchar_transform(internal),
+    FROM SQL WITH FUNCTION prsd_lextype(internal),
     TO SQL WITH FUNCTION int4recv(internal));
 NOTICE:  DDL test: type simple, tag CREATE TRANSFORM
 DROP TRANSFORM FOR int LANGUAGE SQL;

src/test/modules/test_ddl_deparse/sql/create_transform.sql

@@ -6,9 +6,11 @@
 -- The function FROM SQL should have internal as single argument as well
 -- as return type. The function TO SQL should have as single argument
 -- internal and as return argument the datatype of the transform done.
--- pl/plpgsql does not authorize the use of internal as data type.
+-- We choose some random built-in functions that have the right signature.
+-- This won't actually be used, because the SQL function language
+-- doesn't implement transforms (there would be no point).
 CREATE TRANSFORM FOR int LANGUAGE SQL (
-    FROM SQL WITH FUNCTION varchar_transform(internal),
+    FROM SQL WITH FUNCTION prsd_lextype(internal),
     TO SQL WITH FUNCTION int4recv(internal));
 
 DROP TRANSFORM FOR int LANGUAGE SQL;

src/test/regress/expected/object_address.out

@@ -33,8 +33,10 @@ CREATE SERVER "integer" FOREIGN DATA WRAPPER addr_fdw;
 CREATE USER MAPPING FOR regress_addr_user SERVER "integer";
 ALTER DEFAULT PRIVILEGES FOR ROLE regress_addr_user IN SCHEMA public GRANT ALL ON TABLES TO regress_addr_user;
 ALTER DEFAULT PRIVILEGES FOR ROLE regress_addr_user REVOKE DELETE ON TABLES FROM regress_addr_user;
+-- this transform would be quite unsafe to leave lying around,
+-- except that the SQL language pays no attention to transforms:
 CREATE TRANSFORM FOR int LANGUAGE SQL (
-	FROM SQL WITH FUNCTION varchar_transform(internal),
+	FROM SQL WITH FUNCTION prsd_lextype(internal),
 	TO SQL WITH FUNCTION int4recv(internal));
 CREATE PUBLICATION addr_pub FOR TABLE addr_nsp.gentable;
 CREATE SUBSCRIPTION addr_sub CONNECTION '' PUBLICATION bar WITH (connect = false, slot_name = NONE);

src/test/regress/sql/object_address.sql

@@ -36,8 +36,10 @@ CREATE SERVER "integer" FOREIGN DATA WRAPPER addr_fdw;
 CREATE USER MAPPING FOR regress_addr_user SERVER "integer";
 ALTER DEFAULT PRIVILEGES FOR ROLE regress_addr_user IN SCHEMA public GRANT ALL ON TABLES TO regress_addr_user;
 ALTER DEFAULT PRIVILEGES FOR ROLE regress_addr_user REVOKE DELETE ON TABLES FROM regress_addr_user;
+-- this transform would be quite unsafe to leave lying around,
+-- except that the SQL language pays no attention to transforms:
 CREATE TRANSFORM FOR int LANGUAGE SQL (
-	FROM SQL WITH FUNCTION varchar_transform(internal),
+	FROM SQL WITH FUNCTION prsd_lextype(internal),
 	TO SQL WITH FUNCTION int4recv(internal));
 CREATE PUBLICATION addr_pub FOR TABLE addr_nsp.gentable;
 CREATE SUBSCRIPTION addr_sub CONNECTION '' PUBLICATION bar WITH (connect = false, slot_name = NONE);