[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299010
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