Implement PreprocessingRecord's and LazyVector's iterators on top of iterator_adaptor_base

This basically creates a wrapper around an 'int' that poses as an iterator.
While that looks a bit counter-intuitive it works just fine because iterator
operations and basic integer arithmetic works in exactly the same way.

Remove the manual integer wrapping code and reduce the reliance on iterator
internals in the implementation. No functionality change intended.

llvm-svn: 232322