Driver: Use pointee_iterator rather than iterating over unique_ptrs

There's probably never a good reason to iterate over unique_ptrs. This
lets us use range-for and say Job.foo instead of (*it)->foo in a few
places.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218938 91177308-0d34-0410-b5e6-96231b3b80d8