libauto: more obscene hackery to make it work on LLVM 4

This sort of thing is going to get revamped to be less hackish soon but
for now I want it to work. In this particular case, libc++ 4 (and maybe
earlier) gets very upset if we're imprecise about our const markers, and
I guess libauto was careless. This fixes it (PtrPtrMap) to be correct.