Use range-based for loops to iterate over file nodes.

Use range-based for loops to iterate over file nodes.

I converted them to non-range-based loops in r226883 and r226893
because at that moment File::parse() may have side effects and
may update the vector that the reference returned from
LinkingContext::nodes().

Now File::parse() is free from side effects. We can use range-based
loops again.

llvm-svn: 231321