pgsql: If an index depends on no columns of its table, give it a

Log Message:
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If an index depends on no columns of its table, give it a dependency on the
whole table instead, to ensure that it goes away when the table is dropped.
Per bug #3723 from Sam Mason.

Backpatch as far as 7.4; AFAICT 7.3 does not have the issue, because it doesn't
have general-purpose expression indexes and so there must be at least one
column referenced by an index.

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/catalog:
        index.c (r1.261.2.2 -> r1.261.2.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c?r1=1.261.2.2&r2=1.261.2.3)