Need to do SPI_push/SPI_pop around expression evaluation in plpgsql.

We must do this in case the expression evaluation results in calling
another plpgsql function (or, really, anything using SPI).  I missed
the need for this when I converted exec_cast_value() from doing a
simple InputFunctionCall() to doing ExecEvalExpr() in commit 1345cc67b.
There is a SPI_push_conditional in InputFunctionCall(), so that there
was no bug before that.

Per bug #14414 from Marcos Castedo.  Add a regression test based on his
example, which was that a plpgsql function in a domain check constraint
didn't work when assigning to a domain-type variable within plpgsql.

Report: <20161106010947.1387.66380@wrigleys.postgresql.org>

src/test/regress/sql/plpgsql.sql

@@ -4428,3 +4428,25 @@ exception when others then
   null; -- do nothing
 end;
 $$;
+
+-- Test use of plpgsql in a domain check constraint (cf. bug #14414)
+
+create function plpgsql_domain_check(val int) returns boolean as $$
+begin return val > 0; end
+$$ language plpgsql immutable;
+
+create domain plpgsql_domain as integer check(plpgsql_domain_check(value));
+
+do $$
+declare v_test plpgsql_domain;
+begin
+  v_test := 1;
+end;
+$$;
+
+do $$
+declare v_test plpgsql_domain := 1;
+begin
+  v_test := 0;  -- fail
+end;
+$$;