[lworld] Change Javac's diagnostic emission strategy for cyclic.value.type.membership

See http://mail.openjdk.java.net/pipermail/valhalla-dev/2018-February/003815.html

Presently Javac's checks for cycles in value type membership is keyed off of the __ByValue modifier. In the L-World exploration: "fields without the ACC_FLATTENABLE flag set are handled as regular fields, even if their type is a value class type: no pre-loading, no cycle detection."

Javac should accordingly change its emission of cyclic.value.type.membership diagnostic.