[Objective-C] Fix non-determinism in clang

Summary: Iteration of the unordered Ivars causes objc-modern-metadata-visibility.mm (uncovered by reverse iterating SmallPtrSet).

Reviewers: dblaikie, davide, rsmith

Reviewed By: dblaikie

Subscribers: cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D34860

llvm-svn: 307296