Renamed function IsPotentialUse to IsPotentialRetainableObjPtr.

This name change does the following:

1. Causes the function name to use proper ARC terminology.
2. Makes it clear what the function truly does.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173609 91177308-0d34-0410-b5e6-96231b3b80d8