Fix pointer and reference traits for Frozen2 range iterators
Fix pointer and reference traits for Frozen2 range iterators

Summary:
This fixes the inconsistency between the types returned by
`iterator_traits<>::{reference,pointer}` (which is currently `void`)
and the actual return types of `operator*` and `operator->`, which
break some algorithms on iterators.

Reviewed By: luciang, philippv

Differential Revision: D4005787

fbshipit-source-id: 6465882c717ea806ed773527fd8892acdd77787a

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