Give sentinel traits the right to determine the policy where the sentinel is kept.

Give sentinel traits the right to determine the policy where the sentinel is kept.
This should result in less indirect memory accesses, less dead writes and tighter code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66061 91177308-0d34-0410-b5e6-96231b3b80d8