DebugInfo: Update for LLVM API change in DIBuilder (r235111)

LLVM r235111 changed the `DIBuilder` API to stop using `DIDescriptor`
and its subclasses.  Rolled into this was some tightening up of types:

  - Scopes: `DIDescriptor` => `MDScope*`.
  - Generic debug nodes: `DIDescriptor` => `DebugNode*`.
  - Subroutine types: `DICompositeType` => `MDSubroutineType*`.
  - Composite types: `DICompositeType` => `MDCompositeType*`.

Note that `DIDescriptor` wraps `MDNode`, and `DICompositeType` wraps
`MDCompositeTypeBase`.

It's this new type strictness that requires changes here.

llvm-svn: 235112