This will be used to provide a safe overload of `std::vector::erase` in Swift.
`std::vector::erase` is not currently imported into Swift because it returns a C++ iterator.
rdar://113704853
This is currently a no-op since we don't auto-conform custom types to any kind of `MutableCollection` protocols: `UnsafeCxxMutableInputIterator` is only used for `std::map` conformance to `CxxDictionary` in the overlay. However, this will be useful in the future if we start conforming mutable C++ containers to certain Swift protocols: that way containers that use raw non-const pointers as iterators would be treated as mutable in Swift.
This is an inheritor of the existing `UnsafeCxxInputIterator` protocol, with the only difference being the ability to mutate `var pointee` via a non-const `operator*()`.
This is needed to support mutable subscripts for `std::map` via `CxxDictionary`.
rdar://105399019
Since the unsafe iterator types are now used throughout the overlay, not just by `CxxSequence` and `CxxRandomAccessCollection`, let's move them to a separate file.
