Use SmallPtrSet instead of SmallSet in places where we iterate over the set.

SmallSet forwards to SmallPtrSet for pointer types. SmallPtrSet supports iteration, but a normal SmallSet doesn't. So if it wasn't for the forwarding, this wouldn't work.

These places were found by hiding the begin/end methods in the SmallSet forwarding.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@334339 91177308-0d34-0410-b5e6-96231b3b80d8