pgsql: Fix for dropped columns in a partitioned table's default partiti

Fix for dropped columns in a partitioned table's default partition

We forgot to map column numbers to/from the default partition for
various operations, leading to valid cases failing with spurious
errors, such as
ERROR: attribute N of type some_partition has been dropped

It was also possible that the search for conflicting rows in the default
partition when attaching another partition would fail to detect some.
Secondarily, it was also possible that such a search should be skipped
(because the constraint was implied) but wasn't.

Fix all this by mapping column numbers when necessary.

Reported by: Daniel Wilches
Author: Amit Langote
Discussion: https://postgr.es/m/15873-8c61945d6b3ef87c@postgresql.org

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/43085a4f693e35d674a9965c484e4e87f18f6d29

Modified Files
--------------
src/backend/commands/tablecmds.c | 7 +++++++
src/backend/partitioning/partbounds.c | 24 +++++++++++++++++++-----
src/test/regress/expected/alter_table.out | 9 ++++++++-
src/test/regress/sql/alter_table.sql | 9 ++++++++-
4 files changed, 42 insertions(+), 7 deletions(-)