pgsql: Fix overly tense optimization of PLpgSQL_func_hashkey: we must

Log Message:
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Fix overly tense optimization of PLpgSQL_func_hashkey: we must represent
the isTrigger state explicitly, not rely on nonzero-ness of trigrelOid
to indicate trigger-hood, because trigrelOid will be left zero when compiling
for validation. The (useless) function hash entry built by the validator
was able to match an ordinary non-trigger call later in the same session,
thereby bypassing the check that is supposed to prevent such a call.
Per report from Alvaro.

It might be worth suppressing the useless hash entry altogether, but
that's a bigger change than I want to consider back-patching.

Back-patch to 8.0. 7.4 doesn't have the problem because it doesn't
have validation mode.

Modified Files:
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pgsql/src/pl/plpgsql/src:
pl_comp.c (r1.130 -> r1.131)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_comp.c?r1=1.130&r2=1.131)
plpgsql.h (r1.100 -> r1.101)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h?r1=1.100&r2=1.101)