pgsql: Repair unsafe use of shared typecast-lookup table in plpgsql DO

Repair unsafe use of shared typecast-lookup table in plpgsql DO blocks.

DO blocks use private simple_eval_estates to avoid intra-transaction memory
leakage, cf commit c7b849a89.  I had forgotten about that while writing
commit 0fc94a5ba, but it means that expression execution trees created
within a DO block disappear immediately on exiting the DO block, and hence
can't safely be linked into plpgsql's session-wide cast hash table.
To fix, give a DO block a private cast hash table to go with its private
simple_eval_estate.  This is less efficient than one could wish, since
DO blocks can no longer share any cast lookup work with other plpgsql
execution, but it shouldn't be too bad; in any case it's no worse than
what happened in DO blocks before commit 0fc94a5ba.

Per bug #13571 from Feike Steenbergen.  Preliminary analysis by
Oleksandr Shulgin.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/83604cc42353b6c0de2a3f3ac31f94759a9326ae

Modified Files
--------------
src/pl/plpgsql/src/pl_exec.c          |   79 +++++++++++++++++++++------------
src/pl/plpgsql/src/plpgsql.h          |    4 ++
src/test/regress/expected/plpgsql.out |    7 +++
src/test/regress/sql/plpgsql.sql      |    9 ++++
4 files changed, 71 insertions(+), 28 deletions(-)