DebugInfo: Remove DIDescriptor from the DIBuilder API

As a step toward killing `DIDescriptor` and its subclasses, remove it
from the `DIBuilder` API.  Replace the subclasses with appropriate
pointers from the new debug info hierarchy.  There are a couple of
possible surprises in type choices for out-of-tree frontends:

  - Subroutine types: `MDSubroutineType`, not `MDCompositeTypeBase`.
  - Composite types: `MDCompositeType`, not `MDCompositeTypeBase`.
  - Scopes: `MDScope`, not `MDNode`.
  - Generic debug info nodes: `DebugNode`, not `MDNode`.

This is part of PR23080.

llvm-svn: 235111