Basic support for formatting for-loops.

We used to not really format them. Now we do:

  for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
                                        SE = BB->succ_end();
       SI != SE; ++SI) {

This is just one example and I am sure we still mess some of them up, but it
is a step forward.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170899 91177308-0d34-0410-b5e6-96231b3b80d8