While private and protected fields coming from C++ cannot be accessed from Swift, they can affect Swift typechecking.
For instance, the Swift typechecker mechanism that adds implicit `Sendable` conformances works by iterating over all of the struct's fields and checking whether all of them are `Sendable`. This logic was broken for C++ types with private fields, since they were never accounted for. This resulted in erroneous implicit `Sendable` confromances being added.
Same applies for `BitwiseCopyable`.
In addition to this, ClangImporter used to mistakenly mark all C++ structs that have private fields as types with unreferenceable storage, which hampered optimizations.
As a side effect of this change, we now also provide a better diagnostic when someone tries to access a private C++ field from Swift.
rdar://134430857
If a C++ type `Derived` inherits from `Base` privately, the public methods from `Base` should not be callable on an instance of `Derived`. However, C++ supports exposing such methods via a using declaration: `using MyPrivateBase::myPublicMethod;`.
MSVC started using this feature for `std::optional` which means Swift doesn't correctly import `var pointee: Pointee` for instantiations of `std::optional` on Windows. This prevents the automatic conformance to `CxxOptional` from being synthesized.
rdar://114282353 / resolves https://github.com/apple/swift/issues/68068
