[Objective-C] Fix __weak type traits with -fobjc-weak

Summary: Similar to ARC, in ObjCWeak Objective-C object pointers qualified with a weak lifetime are not POD or trivial types. Update the type trait code to reflect this. Copy and adapt the arc-type-traits.mm test case to verify correctness.

Reviewers: rsmith, doug.gregor, rjmccall

Reviewed By: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D31004

llvm-svn: 299010