Add new OID alias type regdatabase.

 Add new OID alias type regdatabase.
 
 This provides a convenient way to look up a database's OID.  For
 example, the query
 
     SELECT * FROM pg_shdepend
     WHERE dbid = (SELECT oid FROM pg_database
                   WHERE datname = current_database());
 
 can now be simplified to
 
     SELECT * FROM pg_shdepend
     WHERE dbid = current_database()::regdatabase;
 
 Like the regrole type, regdatabase has cluster-wide scope, so we
 disallow regdatabase constants from appearing in stored
 expressions.
 
 Bumps catversion.
 
 Author: Ian Lawrence Barwick <barwick@gmail.com>
 Reviewed-by: Greg Sabino Mullane <htamfids@gmail.com>
 Reviewed-by: Jian He <jian.universality@gmail.com>
 Reviewed-by: Fabrízio de Royes Mello <fabriziomello@gmail.com>
 Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
 Discussion: https://postgr.es/m/aBpjJhyHpM2LYcG0%40nathan