Unavailable enum elements cannot be instantiated at runtime without invoking
UB. Therefore the optimizer can consider a basic block unreachable if its only
predecessor is a block that terminates in a switch instruction matching an
unavailable enum element. Furthermore, removing the switch instruction cases
that refer to unavailable enum elements is _mandatory_ when
`-unavailable-decl-optimization=complete` is specified because otherwise
lowered IR for these instructions could refer to enum tag accessors that will
not be lowered, resulting in a failure during linking.
Resolves rdar://113872720.
