[debug-early] reuse variable DIEs and fix their context

[Jan Hubicka]

> It looks like java aliases are still using assemble_alias directly;

assemble_alias dispatch to code adding alias pair to callgarph, so that should
be safe.  My understanding is that Aldys concern is that creating the aliases
early leads to undefined symbol because we produce aliases that never get their
function bodies output by FE.

> switching to using same_body aliases like thunks and such should
> handle the issue.

Yes, I think producing java alias at the same time we produce same_body alias should just work
(not that I would like the same body alias machinery - it is major pain to see these appearing
at random time from DECL_ASSEMBLER_NAME hook)

Honza
> 
> Jason