[mach-o] add llvm_unreachable

llvm-svn: 194172
diff --git a/lld/lib/ReaderWriter/MachO/MachONormalizedFileFromAtoms.cpp b/lld/lib/ReaderWriter/MachO/MachONormalizedFileFromAtoms.cpp
index d15e20e..4f1d529 100644
--- a/lld/lib/ReaderWriter/MachO/MachONormalizedFileFromAtoms.cpp
+++ b/lld/lib/ReaderWriter/MachO/MachONormalizedFileFromAtoms.cpp
@@ -496,6 +496,7 @@
   case Atom::scopeGlobal:
     return N_EXT;
   }
+  llvm_unreachable("Unknown scope");
 }
 
 bool Util::AtomSorter::operator()(const AtomAndIndex &left,