pgsql: Fix confusion about event trigger vs. plain function in plpgsql.

Fix confusion about event trigger vs. plain function in plpgsql.

The function hash table keys made by compute_function_hashkey() failed
to distinguish event-trigger call context from regular call context.
This meant that once we'd successfully made a hash entry for an event
trigger (either by validation, or by normal use as an event trigger),
an attempt to call the trigger function as a plain function would
find this hash entry and thereby bypass the you-can't-do-that check in
do_compile().  Thus we'd attempt to execute the function, leading to
strange errors or even crashes, depending on function contents and
server version.

To fix, add an isEventTrigger field to PLpgSQL_func_hashkey,
paralleling the longstanding infrastructure for regular triggers.
This fits into what had been pad space, so there's no risk of an ABI
break, even assuming that any third-party code is looking at these
hash keys.  (I considered replacing isTrigger with a PLpgSQL_trigtype
enum field, but felt that that carried some API/ABI risk.  Maybe we
should change it in HEAD though.)

Per bug #16266 from Alexander Lakhin.  This has been broken since
event triggers were invented, so back-patch to all supported branches.

Discussion: https://postgr.es/m/16266-fcd7f838e97ba5d4@postgresql.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/32aa14d40c7ce12e648c9f88fe22fa0146bee3b4

Modified Files
--------------
src/pl/plpgsql/src/pl_comp.c                | 15 +++++++++++----
src/pl/plpgsql/src/plpgsql.h                |  5 +++--
src/test/regress/expected/event_trigger.out |  4 ++++
src/test/regress/sql/event_trigger.sql      |  3 +++
4 files changed, 21 insertions(+), 6 deletions(-)