pgsql: Restore the portal-level snapshot for simple expressions, too.

Restore the portal-level snapshot for simple expressions, too.

Commits 84f5c2908 et al missed the need to cover plpgsql's "simple
expression" code path. If the first thing we execute after a
COMMIT/ROLLBACK is one of those, rather than a full-fledged SPI command,
we must explicitly do EnsurePortalSnapshotExists() to make sure we have
an outer snapshot. Note that it wouldn't be good enough to just push a
snapshot for the duration of the expression execution: what comes back
might be toasted, so we'd better have a snapshot protecting it.

The test case demonstrating this fact cheats a bit by marking a SQL
function immutable even though it fetches from a table. That's
nothing that users haven't been seen to do, though.

Per report from Jim Nasby. Back-patch to v11, like the previous fix.

Discussion: https://postgr.es/m/378885e4-f85f-fc28-6c91-c4d1c080bf26@amazon.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/29d5d5761aa8695e1226f1f76b7f76a2a4b195a0

Modified Files
--------------
src/pl/plpgsql/src/expected/plpgsql_transaction.out | 18 ++++++++++++++++++
src/pl/plpgsql/src/pl_exec.c | 10 ++++++++++
src/pl/plpgsql/src/sql/plpgsql_transaction.sql | 21 +++++++++++++++++++++
3 files changed, 49 insertions(+)