Add precondition to prevent access out of bounds

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

Author: susmonteiro

stdlib/public/Cxx/CxxRandomAccessCollection.swift

@@ -44,6 +44,7 @@ extension CxxRandomAccessCollection {
       // Not using CxxIterator here to avoid making a copy of the collection.
       var rawIterator = __beginUnsafe()
       rawIterator += RawIterator.Distance(index)
+      precondition(__endUnsafe() - rawIterator > 0, "C++ iterator access out of bounds")  
       yield rawIterator.pointee
     }
   }