pgsql: Fix FK checks of TRUNCATE involving partitioned tables

Fix FK checks of TRUNCATE involving partitioned tables

When truncating a table that is referenced by foreign keys in
partitioned tables, the check to ensure the referencing table are also
truncated spuriously failed.  This is because it was relying on
relhastriggers as a proxy for the table having FKs, and that's wrong for
partitioned tables.  Fix it to consider such tables separately.  There
may be a better way ... but this code is pretty inefficient already.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Michael Paquiër <michael@paquier.xyz>
Discussion: https://postgr.es/m/20180711000624.zmeizicibxeehhsg@alvherre.pgsql

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/727196ced2423008bb25d7775c4e3b7fca063c8b

Modified Files
--------------
src/backend/catalog/heap.c             |  7 +++-
src/backend/commands/tablecmds.c       |  2 +-
src/test/regress/expected/truncate.out | 75 ++++++++++++++++++++++++++++++++++
src/test/regress/sql/truncate.sql      | 47 +++++++++++++++++++++
4 files changed, 128 insertions(+), 3 deletions(-)