pgsql: Prevent altering partitioned table's rowtype, if it's used elsew

Prevent altering partitioned table's rowtype, if it's used elsewhere.

We disallow altering a column datatype within a regular table,
if the table's rowtype is used as a column type elsewhere,
because we lack code to go around and rewrite the other tables.
This restriction should apply to partitioned tables as well, but it
was not checked because ATRewriteTables and ATPrepAlterColumnType
were not on the same page about who should do it for which relkinds.

Per bug #17351 from Alexander Lakhin. Back-patch to all supported
branches.

Discussion: https://postgr.es/m/17351-6db1870f3f4f612a@postgresql.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ccc7c3ad86b67cd85805824d40782fa5b92b1fc5

Modified Files
--------------
src/backend/commands/tablecmds.c | 7 +++----
src/test/regress/expected/alter_table.out | 23 ++++++++++++++++++-----
src/test/regress/sql/alter_table.sql | 19 +++++++++++++++----
3 files changed, 36 insertions(+), 13 deletions(-)