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

src/backend/utils/adt/selfuncs.c

@@ -4619,6 +4619,7 @@ convert_to_scalar(Datum value, Oid valuetypid, Oid collid, double *scaledvalue,
 		case REGDICTIONARYOID:
 		case REGROLEOID:
 		case REGNAMESPACEOID:
+		case REGDATABASEOID:
 			*scaledvalue = convert_numeric_to_scalar(value, valuetypid,
 													 &failure);
 			*scaledlobound = convert_numeric_to_scalar(lobound, boundstypid,
@@ -4751,6 +4752,7 @@ convert_numeric_to_scalar(Datum value, Oid typid, bool *failure)
 		case REGDICTIONARYOID:
 		case REGROLEOID:
 		case REGNAMESPACEOID:
+		case REGDATABASEOID:
 			/* we can treat OIDs as integers... */
 			return (double) DatumGetObjectId(value);
 	}