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.

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