Summary: Instead of accessing the fields directly, use accessor functions to provide access to the DIERef components. This allows us to decouple the external interface, from the internal representation. The external interface can use llvm::Optional and similar goodies, while the data can still be stored internally in a more compact representation.
I also document the purpose of the existing DIERef fields.
The main motivation for this change is a need to introduce an additional field to the DIERef class, but I believe the change has its own merit.