[cxx-interop] Change the hierarchy of CxxSequence protocols

This makes `CxxConvertibleToCollection` the base protocol in the hierarchy. Both `CxxSequence` and `CxxRandomAccessCollection` now inherit from `CxxConvertibleToCollection`.

Author: egorzhdan

stdlib/public/Cxx/CxxSequence.swift

@@ -65,9 +65,9 @@ extension Optional: UnsafeCxxInputIterator where Wrapped: UnsafeCxxInputIterator
 /// This requires the C++ sequence type to define const methods `begin()` and
 /// `end()` which return input iterators into the C++ sequence. The iterator
 /// types must conform to `UnsafeCxxInputIterator`.
-public protocol CxxSequence<Element>: Sequence {
+public protocol CxxSequence<Element>: CxxConvertibleToCollection, Sequence {
   override associatedtype Element
-  associatedtype RawIterator: UnsafeCxxInputIterator
+  override associatedtype RawIterator: UnsafeCxxInputIterator
     where RawIterator.Pointee == Element
   override associatedtype Iterator = CxxIterator<Self>