Fixed a bug related to SQL statements in PL/pgSQL functions.

Fixed a bug related to SQL statements in PL/pgSQL functions.

At least since 9.1, true query strings of SQL statements executed in
PL/pgSQL functions were found that were not obtained correctly by the
previous implement, it is because that PLpgSQL_stmt variable did not
have proper query string to be read for hints. Instead, it is changed
to read them from the top of error_context_stack in
pg_hint_plan_planner(). This change made a slight difference in
behavior which doesn't affect its work so a part of the regtest was
also changed. And added some regression tests for it.