pgsql: refactor: re-add ATExecAlterChildConstr()

refactor: re-add ATExecAlterChildConstr()

ATExecAlterChildConstr() was removed in commit 80d7f990496, but it is
needed in some subsequent patches for the NOT ENFORCED feature, to
recurse over child constraints.  This adds it back in slightly altered
form.

Author: Amul Sul <amul.sul@enterprisedb.com>
Reviewed-by: Alexandra Wang <alexandra.wang.oss@gmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/CAAJ_b962c5AcYW9KUt_R_ER5qs3fUGbe4az-SP-vuwPS-w-AGA%40mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/64224a834ce4c899e878db27b14ea3412a6ddbed

Modified Files
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src/backend/commands/tablecmds.c | 84 ++++++++++++++++++++++++++--------------
1 file changed, 54 insertions(+), 30 deletions(-)