SCC: Change clients to use const, NFC

It's fishy to be changing the `std::vector<>` owned by the iterator, and
no one actual does it, so I'm going to remove the ability in a
subsequent commit.  First, update the users.

<rdar://problem/14292693>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207252 91177308-0d34-0410-b5e6-96231b3b80d8