pgsql: Rearrange "add column" logic to merge columns at exec time.

Rearrange "add column" logic to merge columns at exec time.

The previous coding set attinhcount too high in some cases, resulting in
an undumpable, undroppable column.  Per bug #5856, reported by Naoya
Anzai.  See also commit 31b6fc06d83c6de3644c8f2921eb7de0eb92fac3, which
fixes a similar bug in ALTER TABLE .. ADD CONSTRAINT.

Patch by Noah Misch.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6c5723998594dffa5d47c3cf8c96ccf89c033aae

Modified Files
--------------
src/backend/commands/tablecmds.c          |  161 ++++++++++++++++-------------
src/include/nodes/parsenodes.h            |    2 +
src/test/regress/expected/alter_table.out |   17 +++
src/test/regress/sql/alter_table.sql      |   12 ++
4 files changed, 119 insertions(+), 73 deletions(-)