Teach DeadStoresChecker about attribute objc_precise_lifetime.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199277 91177308-0d34-0410-b5e6-96231b3b80d8
ObjectiveC. Improve on diagnostics per Jordan's feedback.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199278 91177308-0d34-0410-b5e6-96231b3b80d8