ELF: Make findAbsoluteAtom return AtomLayout* instead of an iterator.

All calls of findAbsoluteAtoms seem a bit awkward because of the type
of the function. It semantically returns a pointer to an AtomLayout or
nothing, so I made the function return AtomLayout*.

In this patch, I also expanded some "auto"s because their actual type
were not obvious in their contexts.

llvm-svn: 233769