MachO: stop iterating past the end of an array

When trying to map atom types to sections, we were iterating through an array
until we hit a sentinel value. There's no need for such dances when range-based
for loops are available.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@212035 91177308-0d34-0410-b5e6-96231b3b80d8