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.

src/test/regress/expected/plpgsql.out

@@ -4764,6 +4764,13 @@ commit;
 drop function cast_invoker(integer);
 drop function sql_to_date(integer) cascade;
 NOTICE:  drop cascades to cast from integer to date
+-- Test handling of cast cache inside DO blocks
+-- (to check the original crash case, this must be a cast not previously
+-- used in this session)
+begin;
+do $$ declare x text[]; begin x := '{1.23, 4.56}'::numeric[]; end $$;
+do $$ declare x text[]; begin x := '{1.23, 4.56}'::numeric[]; end $$;
+end;
 -- Test for consistent reporting of error context
 create function fail() returns int language plpgsql as $$
 begin

src/test/regress/sql/plpgsql.sql

@@ -3836,6 +3836,15 @@ commit;
 drop function cast_invoker(integer);
 drop function sql_to_date(integer) cascade;
 
+-- Test handling of cast cache inside DO blocks
+-- (to check the original crash case, this must be a cast not previously
+-- used in this session)
+
+begin;
+do $$ declare x text[]; begin x := '{1.23, 4.56}'::numeric[]; end $$;
+do $$ declare x text[]; begin x := '{1.23, 4.56}'::numeric[]; end $$;
+end;
+
 -- Test for consistent reporting of error context
 
 create function fail() returns int language plpgsql as $$