Fix warnings in build on clang-x86_64-freebsd buildbot.

llvm-svn: 66344
diff --git a/clang/lib/Driver/Arg.cpp b/clang/lib/Driver/Arg.cpp
index 52437e7..16efda8 100644
--- a/clang/lib/Driver/Arg.cpp
+++ b/clang/lib/Driver/Arg.cpp
@@ -145,4 +145,5 @@
                                            unsigned N) const {
   assert(N < getNumValues() && "Invalid index.");
   assert(0 && "FIXME: Implement");
+  return 0;
 }