SE-0155 makes an empty associated value list in an enum element
declaration illegal. Warn about this in legacy Swift mode, and reject
it otherwise. This offers two fixes:
1) Remove the empty associated value list
2) Insert a 'Void' associated value
We can still produce an undefined address for the empty value, even though we considered them as no-payload cases for layout purposes. Fixes rdar://problem/21515490.
Swift SVN r30490