Revert "Repace SmallPtrSet with SmallPtrSetImpl in function arguments to avoid needing to mention the size."

Getting a weird buildbot failure that I need to investigate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215870 91177308-0d34-0410-b5e6-96231b3b80d8