Add forwarding `getFilename()` and `getDirectory()` accessors to nodes
in the new hierarchy that define a `getFile()`. Use that to
re-implement existing functionality in the `DIDescriptor` hierarchy.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234671 91177308-0d34-0410-b5e6-96231b3b80d8