Revert "Remove useless unreachable. Switch coverage already gives us this. NFC"

This reverts commit r264945.

The commit only removed an unreachable in a method with a covered switch, but
GCC is likely to warn on this, and the coding standards recommend just leaving
in the unreachable.

llvm-svn: 264983

lld/lib/ReaderWriter/MachO/MachONormalizedFileFromAtoms.cpp

@@ -865,6 +865,7 @@ llvm::Error Util::getSymbolTableRegion(const DefinedAtom* atom,
     }
     break;
   }
+  llvm_unreachable("atom->scope() unknown enum value");
 }
 
 llvm::Error Util::addSymbols(const lld::File &atomFile,