pgsql: Prevent a rowtype from being included in itself via a range.

Prevent a rowtype from being included in itself via a range.

We probably should have thought of this case when ranges were added,
but we didn't.  (It's not the fault of commit eb51af71f, because
ranges didn't exist then.)

It's an old bug, so back-patch to all supported branches.

Discussion: https://postgr.es/m/7782.1577051475@sss.pgh.pa.us

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/354d913f9745c4ce39c891d9a73b0b2535fd9c4c

Modified Files
--------------
src/backend/catalog/heap.c               | 9 +++++++++
src/test/regress/expected/rangetypes.out | 3 +++
src/test/regress/sql/rangetypes.sql      | 3 +++
3 files changed, 15 insertions(+)