BUG #15718: DROP TABLE fails if it's partitioned by a key of a deleted enum

The following bug has been logged on the website:

Bug reference: 15718
Logged by: Alex
Email address: pisarenco(dot)a(at)gmail(dot)com
PostgreSQL version: 11.2
Operating system: All Linux
Description:

Code to replicate the issue:
https://pastebin.com/rc8q35Qj

This happened to our ETL processes that re-create the staging data on each
run. Enums are defining partition keys for some tables, that then flow into
tables in another schema. When the schema containing that enum gets dropped,
the columns that use it are also dropped. If those columns are also
partition keys, it ends up in a situation where it's not possible to drop
the parent table. Failure with ERROR: cache lookup failed for type 0.

Issue has been observed at least since Postgresql 10, but just got
successfully replicated on minimal code.