CGDebugInfo: Use DIBuilder API for self-referencing DICompositeTypes

Use new `DIBuilder` API from LLVM r224482 to mutate `DICompositeType`s,
rather than changing them directly.  This allows `DIBuilder` to track
otherwise orphaned cycles when `CollectContainingType()` creates a
self-reference.

Fixes PR21941.

llvm-svn: 224483