This makes use of the new PATypeHolder's to keep types from being deleted
prematurely, instead of the wierd "self reference" garbage. This is easier
to understand and more efficient as well.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8834 91177308-0d34-0410-b5e6-96231b3b80d8