pgsql: Fix ALTER TABLE error message

Fix ALTER TABLE error message

This bogus error message was introduced in 2013 by commit f177cbfe676d,
because of misunderstanding the processCASbits() API; at the time, no
test cases were added that would be affected by this change. Only in
ca87c415e2fc was one added (along with a couple of typos), with an XXX
note that the error message was bogus. Fix the whole, add some test
cases.

Backpatch all the way back.

Reviewed-by: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Discussion: https://postgr.es/m/[email protected]

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0f354e0805ed5df05e02f60c6998a7b9df14b4b6

Modified Files
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src/backend/parser/gram.y | 2 +-
src/test/regress/expected/foreign_key.out | 6 +++++-
src/test/regress/sql/foreign_key.sql | 4 +++-
3 files changed, 9 insertions(+), 3 deletions(-)