Add back -Wduplicate-enum which I mistakenly removed.

This was removed with -Wunique-enum, which is still removed.  The
corresponding thread on cfe-comments for that warning is here:

  http://lists.cs.uiuc.edu/pipermail/cfe-dev/2012-September/024224.html

If we get specific user feedback for -Wduplicate-enum we can evaluate
whether or not to keep it.

llvm-svn: 170974