Fix unused variable warning in -Asserts mode.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@67073 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/Driver.cpp b/lib/Driver/Driver.cpp
index e02c283..9805122 100644
--- a/lib/Driver/Driver.cpp
+++ b/lib/Driver/Driver.cpp
@@ -79,6 +79,7 @@
     }
 
     assert(Index > Prev && "Parser failed to consume argument.");
+    (void) Prev;
   }
 
   return Args;