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`.

llvm-svn: 235069
diff --git a/llvm/examples/Kaleidoscope/Chapter8/toy.cpp b/llvm/examples/Kaleidoscope/Chapter8/toy.cpp
index 5665c06..21f2eb1 100644
--- a/llvm/examples/Kaleidoscope/Chapter8/toy.cpp
+++ b/llvm/examples/Kaleidoscope/Chapter8/toy.cpp
@@ -1232,7 +1232,7 @@
   DISubprogram SP = DBuilder->createFunction(
       FContext, Name, StringRef(), Unit, LineNo,
       CreateFunctionType(Args.size(), Unit), false /* internal linkage */,
-      true /* definition */, ScopeLine, DIDescriptor::FlagPrototyped, false, F);
+      true /* definition */, ScopeLine, DebugNode::FlagPrototyped, false, F);
 
   KSDbgInfo.FnScopeMap[this] = SP;
   return F;