[AArch64] translateLocChar - silence dead code warning

Remove default case from switch and return None after the switch()

llvm/lib/Support/FormatVariadic.cpp

@@ -18,10 +18,8 @@ static Optional<AlignStyle> translateLocChar(char C) {
     return AlignStyle::Center;
   case '+':
     return AlignStyle::Right;
-  default:
-    return None;
   }
-  LLVM_BUILTIN_UNREACHABLE;
+  return None;
 }
 
 bool formatv_object_base::consumeFieldLayout(StringRef &Spec, AlignStyle &Where,