Add new driver warning from r151174 to a warning group.  Please let me know if this isn't the appropriate grouping.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151177 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/clang/Basic/DiagnosticDriverKinds.td b/include/clang/Basic/DiagnosticDriverKinds.td
index 3ada80f..021de88 100644
--- a/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/include/clang/Basic/DiagnosticDriverKinds.td
@@ -106,7 +106,8 @@
   "argument unused during compilation: '%0'">,
   InGroup<DiagGroup<"unused-command-line-argument">>;
 def warn_drv_empty_joined_argument : Warning<
-  "joined argument expects addition arg: '%0'">;
+  "joined argument expects addition arg: '%0'">,
+  InGroup<DiagGroup<"unused-command-line-argument">>;
 def warn_drv_not_using_clang_cpp : Warning<
   "not using the clang preprocessor due to user override">;
 def warn_drv_not_using_clang_cxx : Warning<