[Ada] Implementation of aspects within generic units

[Arnaud Charlet]

> OK. Still trying to reproduce here and trying to figure out blindly what
> could be the cause of this behavior for now.

I could finally reproduce on another machine (i686-linux), so I am now
doing a binary search to find out precisely what change caused this
failure, so no need for other people to do it.

I'll send more info ASAP (pending bootstrap/cpu time).

Arno