[lworld] Add support for ACC_FLATTENABLE flag for fields.

- A field may be declared as a candidate for flattening by encoding the flag value ACC_FLATTENABLE (0x0100)
- During class resolution by the VM, If the field marked as ACC_FLATTENABLE is not in fact a value class, loading throws an IncompatibleClassChangeError - which implies that javac should disallow non value fields from being tagged ACC_FLATTENABLE
- A field flagged as ACC_FLATTENABLE must never store the null reference.
- Check impact of ACC_FLATTENABLE on compiler diagnostic cyclic.value.type.membership: A field declared with the ACC_FLATTENABLE flag set must not have the type of its declaring class, nor refer indirectly to its declaring class through a chain of fields with the ACC_FLATTENABLE flag set.