ObjCGNU: Fix empty v3 protocols being emitted two fields short

Summary:
Protocols that were being referenced but could not be fully realized were being emitted without `properties`/`optional_properties`. Since all v3 protocols must be 9 processor words wide, the lack of these fields is catastrophic for the runtime.

As an example, the runtime cannot know [here](https://github.com/gnustep/libobjc2/blob/master/protocol.c#L73) that `properties` and `optional_properties` are invalid.

Reviewers: rjmccall, theraven

Reviewed By: rjmccall, theraven

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D45305

llvm-svn: 329882