This change adds rudimentary support for C++ ranges in Carbon range-for
loops. Since C++ ranges are exposed through `Core.Iterate`, C++ ranges
have the same limitations as Carbon ranges (e.g. can't return
references).
`Iterate.CursorType` now requires `Destroy`, since types that implement
`CppRangeForIterate` can't be used in range-for loops unless their
cursor type can be verifiably destructible.