pgsql: Avoid scribbling on input node tree in CREATE/ALTER DOMAIN.

Avoid scribbling on input node tree in CREATE/ALTER DOMAIN.

This works fine in the "simple Query" code path; but if the
statement is in the plan cache then it's corrupted for future
re-execution. Apply copyObject() to protect the original
tree from modification, as we've done elsewhere.

This narrow fix is applied only to the back branches. In HEAD,
the problem was fixed more generally by commit 7c337b6b5; but
that changed ProcessUtility's API, so it's infeasible to
back-patch.

Per bug #17053 from Charles Samborski.

Discussion: https://postgr.es/m/931771.1623893989@sss.pgh.pa.us
Discussion: https://postgr.es/m/17053-3ca3f501bbc212b4@postgresql.org

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4b8b3562e1e6e5b4abd5e9326f71af73e98dc604

Modified Files
--------------
src/backend/commands/typecmds.c | 21 ++++++++++++++-------
1 file changed, 14 insertions(+), 7 deletions(-)