pgsql: plpgsql's exec_eval_simple_expr() now has to take responsibility

Log Message:
-----------
plpgsql's exec_eval_simple_expr() now has to take responsibility for
advancing ActiveSnapshot when we are inside a volatile function.
Per example from Gaetano Mendola.  Add a regression test to catch
similar problems in future.

Modified Files:
--------------
    pgsql/src/pl/plpgsql/src:
        pl_exec.c (r1.124 -> r1.125)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.124&r2=1.125)
    pgsql/src/test/regress/expected:
        plpgsql.out (r1.22 -> r1.23)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out.diff?r1=1.22&r2=1.23)
    pgsql/src/test/regress/sql:
        plpgsql.sql (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql.diff?r1=1.17&r2=1.18)