Convert some Args index-based iteration to range-style iteration.

This is better for a number of reasons.  Mostly style, but also:

1) Signed-unsigned comparison warnings disappear since there is
   no loop index.
2) Iterating with the range-for style gives you back an entry
   that has more than just a const char*, so it's more efficient
   and more useful.
3) Makes code safter since the type system enforces that it's
   impossible to index out of bounds.

llvm-svn: 283413