Issue Description:

While migrating from Swift 2 to Swift 3.1, for the Parse iOS SDK, we encountered the following issue:

PFRelation< ObjectType: PFObject*> is defined in Objective-C as a lightweight generic.

When reading the `property_getAttributes` in a PFObject subclass in objective-c, the type encoding is property set to: `@"PFRelation\` therefore our codepath is correct as per the test suite here

when defining it in a swift subclass however, the type encoding looses the PFRelation specialization and is rendered as an `@` which is inconsistent with the behaviour with the objective-c runtime.

ex:

class MyObject: PFObject, PFSubclassing {
  @NSManaged var relation: PFRelation<PFObject>
}

We expect the behaviour to be consistent and the swift compiler to produce the right type encoding, in conformance with the objective-c compiler.

Note: observed as well on Swift 4