step one of moving .dup to library

Looks like Andrei and I had the same idea:

https://github.com/D-Programming-Language/dmd/pull/3364

After this is pulled, the followup will be to remove it from the compiler. Good riddance!

And, in 2.068, remove the internal druntime function adDup(). Keep for the moment so `git bisect` will work.