Show 'AS "?column?"' explicitly when it's important.

ruleutils.c was coded to suppress the AS label for a SELECT output
expression if the column name is "?column?", which is the parser's
fallback if it can't think of something better.  This is fine, and
avoids ugly clutter, so long as (1) nothing further up in the parse
tree relies on that column name or (2) the same fallback would be
assigned when the rule or view definition is reloaded.  Unfortunately
(2) is far from certain, both because ruleutils.c might print the
expression in a different form from how it was originally written
and because FigureColname's rules might change in future releases.
So we shouldn't rely on that.

Detecting exactly whether there is any outer-level use of a SELECT
column name would be rather expensive.  This patch takes the simpler
approach of just passing down a flag indicating whether there *could*
be any outer use; for example, the output column names of a SubLink
are not referenceable, and we also do not care about the names exposed
by the right-hand side of a setop.  This is sufficient to suppress
unwanted clutter in all but one case in the regression tests.  That
seems like reasonable evidence that it won't be too much in users'
faces, while still fixing the cases we need to fix.

Per bug #17486 from Nicolas Lutic.  This issue is ancient, so
back-patch to all supported branches.

Discussion: https://postgr.es/m/17486-1ad6fd786728b8af@postgresql.org

src/test/regress/expected/matview.out

@@ -347,7 +347,7 @@ CREATE VIEW mvtest_vt2 AS SELECT moo, 2*moo FROM mvtest_vt1 UNION ALL SELECT moo
  ?column? | integer |           |          |         | plain   | 
 View definition:
  SELECT mvtest_vt1.moo,
-    2 * mvtest_vt1.moo
+    2 * mvtest_vt1.moo AS "?column?"
    FROM mvtest_vt1
 UNION ALL
  SELECT mvtest_vt1.moo,
@@ -363,7 +363,7 @@ CREATE MATERIALIZED VIEW mv_test2 AS SELECT moo, 2*moo FROM mvtest_vt2 UNION ALL
  ?column? | integer |           |          |         | plain   |              | 
 View definition:
  SELECT mvtest_vt2.moo,
-    2 * mvtest_vt2.moo
+    2 * mvtest_vt2.moo AS "?column?"
    FROM mvtest_vt2
 UNION ALL
  SELECT mvtest_vt2.moo,