LLVM r236120 renamed debug info IR constructs to use a `DI` prefix, now
that the `DIDescriptor` hierarchy has been gone for about a week.  This
commit was generated using the rename-md-di-nodes.sh upgrade script
attached to PR23080, followed by running clang-format-diff.py on the
`lib/` portion of the patch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@236121 91177308-0d34-0410-b5e6-96231b3b80d8