[CIR] Enable support for nested struct members in C++ (#142205)
This enables us to compile C++ code with nested structures. The
necessary support for this was already in place, but we were hitting an
NYI error in a place where the implementation only needed to check for
base classes. Base classes really aren't implemented yet, so the error
is left in place but it is now behind a check for a non-zero number of
bases so that the simple case is unblocked.