DebugInfo: Gut DIDescriptor

DebugInfo: Gut DIDescriptor

PR23080 is almost finished.  With this commit, there's no consequential
API in `DIDescriptor` and its subclasses.  What's left?

  - Default-constructed to `nullptr`.
  - Handy `const_cast<>` (constructed from `const`, but accessors are
    non-`const`).

I think the safe way to catch those is to delete the classes and fix
compile errors.  That'll be my next step, after I delete the `DITypeRef`
(etc.) wrapper around `MDTypeRef`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235069 91177308-0d34-0410-b5e6-96231b3b80d8