[V1] Sanitize unknown enum variant name to avoid collisions.

[RCHowell]

Relevant Issue/Bug

#1679

In the above PR, we've hit a naming conflict with enum "UNKNOWN" and the IS UNKNOWN predicate and truth value. This lead to us believing we should sanitize all UNKNOWN enum variants (or perhaps even make it as part of the base enum abstract class) so that we avoid any additional conflicts. Sanitization makes sense here as change to something like "OTHER" may just be kicking the can down the road.

[alancai98]

Specifically for relevant for this comment -- #1679 (comment).