[SwiftifyImport] Add sizedBy support for OpaquePointer (#78315)
This makes it possible to mark a pointer with __sized_by when the
pointee type definition is not included. The wrapper function has the
same interface as if the parameter were a void pointer, since the stdlib
has no `OpaqueBufferPointer` type.
* use swift-ide-test for checking interop signatures
* add xfail test for Span + Optional combo (Optional requires Escapable)
