pgsql: Fix usage of "tableoid" in GENERATED expressions.

Fix usage of "tableoid" in GENERATED expressions.

We consider this supported (though I've got my doubts that it's a
good idea, because tableoid is not immutable). However, several
code paths failed to fill the field in soon enough, causing such
a GENERATED expression to see zero or the wrong value. This
occurred when ALTER TABLE adds a new GENERATED column to a table
with existing rows, and during regular INSERT or UPDATE on a
foreign table with GENERATED columns.

Noted during investigation of a report from Vitaly Ustinov.
Back-patch to v12 where GENERATED came in.

Discussion: https://postgr.es/m/CAM_DEiWR2DPT6U4xb-Ehigozzd3n3G37ZB1+867zbsEVtYoJww@mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/dfe51ffbe78a16368073c2838c04e603d3d76915

Modified Files
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src/backend/commands/tablecmds.c | 13 ++++++++-----
src/backend/executor/nodeModifyTable.c | 24 ++++++++++++++++++------
src/test/regress/expected/generated.out | 13 ++++++++-----
src/test/regress/sql/generated.sql | 5 ++++-
4 files changed, 38 insertions(+), 17 deletions(-)