This conforms mutable C++ container types, such as `std::vector`, to `MutableCollection` via a new overlay protocol `CxxMutableRandomAccessCollection`.
rdar://134531554
The subscript function from CxxRandomAccessCollection did not perform any bounds-checks.
This means that C++ containers that don't provide the operator[] (or C++ containers used in generic contexts) didn't have bounds-checks.
Fixes rdar://126570011
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.
This makes `CxxConvertibleToCollection` the base protocol in the hierarchy. Both `CxxSequence` and `CxxRandomAccessCollection` now inherit from `CxxConvertibleToCollection`.
This will help us to auto-generate conformances to `CxxRandomAccessCollection`, since synthesized conformances must have all of their witnesses explicitly provided.
This helps to bridge C++ random access collections, such as `std::vector` and `std::string`, to Swift by conforming them to `Swift.RandomAccessCollection`
