Provide an upgrade strategy for dump files containing functions declared

Provide an upgrade strategy for dump files containing functions declared
with OPAQUE.  CREATE LANGUAGE, CREATE TRIGGER, and CREATE TYPE will all
accept references to functions declared with OPAQUE --- but they will
issue a NOTICE, and will modify the function entries in pg_proc to have
the preferred type-safe argument or result types instead of OPAQUE.
Per recent pghackers discussions.