Explain why ACC_bottom should never occur in diagnosing ARC casts.

This is just a clarification on Fariborz's original patch, per e-mail
discussion. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161016 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed