SCC: Provide operator->() through iterator_facade_base

Use the fancy new `iterator_facade_base` to add
`scc_iterator::operator->()`.  Remove other definitions where
`iterator_facade_base` does the right thing.

<rdar://problem/14292693>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207257 91177308-0d34-0410-b5e6-96231b3b80d8