pgsql: Use context with correct lifetime inhypothetical_dense_rank_fin

Use context with correct lifetime in hypothetical_dense_rank_final.

The query lifetime expression context created in
hypothetical_dense_rank_final() was buggily allocated in the calling
memory context. I (Andres) broke that in bf6c614a2f2.

Reported-By: Rajkumar Raghuwanshi
Author: Amit Langote
Discussion:  https://postgr.es/m/CAKcux6kmzWmur5HhA_aU6gYVFu0RLQdgJJ+aC9SLdcOvBSrpfA@mail.gmail.com
Backpatch: 11-

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e60cfcefe635b90bab60e885be689700759a01c6

Modified Files
--------------
src/backend/utils/adt/orderedsetaggs.c   | 10 +++++++++-
src/test/regress/expected/aggregates.out |  9 +++++++++
src/test/regress/sql/aggregates.sql      |  3 +++
3 files changed, 21 insertions(+), 1 deletion(-)