pgsql: Fix unwarranted assumption that a cached rowtype would stick

Log Message:
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Fix unwarranted assumption that a cached rowtype would stick around
for the lifespan of the CreateStmt. Per buildfarm member jaguar.

Modified Files:
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pgsql/src/backend/parser:
parse_utilcmd.c (r2.37 -> r2.38)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_utilcmd.c?r1=2.37&r2=2.38)