Fix isWeaklyIsolated for interfaces

Interfaces are aggregate types, but their FieldTypeTuple is a tuple containing
exactly themselves. This causes an infinite recursion in isWeaklyIsolated.
Since the implementation of an interface is not known at compile time,
isWeaklyIsoleted must be false for interfaces.

Signed-off-by: Luca Niccoli <lultimouomo@gmail.com>