Try to appease win7 bots after r278532 by cleaning up type trait

The HasGetNext type trait was cluttered with a few things it didn't
need.  Try to clean it up, hoping to fix windows bots:
  http://lab.llvm.org:8011/builders/lld-x86_64-win7/builds/38063

I may just have to delete the trait...

llvm-svn: 278537