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 ...