Add concept of invalid value to SysCacheIdentifier

Add concept of invalid value to SysCacheIdentifier

This commit tweaks the generation of the syscache IDs for the enum
SysCacheIdentifier to now include an invalid value, with -1 assigned as
value.  The concept of an invalid syscache ID exists when handling
lookups of a ObjectAddress, based on their set of properties in
ObjectPropertyType.  -1 is used for the case where an object type has no
option for a syscache lookup.

This has been found as independently useful while discussing a switch of
SysCacheIdentifier to a typedef, as we already have places that want to
know about the concept of an invalid value when dealing with
ObjectAddresses.

Reviewed-by: Andreas Karlsson <andreas@proxel.se>
Discussion: https://postgr.es/m/aZQRnmp9nVjtxAHS@paquier.xyz