Speculatively fix some code handling Power64 MachO files

MSVC was warning on a switch containing only default labels.  In this
instance, it looks like it uncovered a real bug.  :)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212062 91177308-0d34-0410-b5e6-96231b3b80d8