Fix unused variable warning in -Asserts mode.

llvm-svn: 67073
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index e02c283..9805122 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -79,6 +79,7 @@
     }
 
     assert(Index > Prev && "Parser failed to consume argument.");
+    (void) Prev;
   }
 
   return Args;