Moved some comments due to the recent refactoring of ObjCARC.

1. Moved a comment from ObjCARCOpts.cpp -> ObjCARCContract.cpp.
2. Removed a comment from ObjCARCOpts.cpp that was already moved to
ObjCARCAliasAnalysis.h/.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174581 91177308-0d34-0410-b5e6-96231b3b80d8