Fix back-branch breakage from ill-advised last-minute commit. 

Tag 8.4.5 

Tag 8.3.12 

Tag 8.2.18 

Tag 8.1.22 

Tag 8.0.26 

Tag 7.4.30 

 tag v9.0.0 ... the big day approaches

Install a data-type-based solution for protecting pg_get_expr(). Since the code underlying pg_get_expr() is not secure against malformed input, and can't practically be made so, we need to prevent miscreants from feeding arbitrary data to it. We can do this securely by declaring pg_get_expr() to take a new datatype "pg_node_tree" and declaring the system catalog columns that hold nodeToString output to be of that type. There is no way at SQL level to create a non-null value of type pg_node_tree. Since the backend-internal operations that fill those catalog columns operate below the SQL level, they are oblivious to the datatype relabeling and don't need any changes.

 tag rc1 ... final stretch ...