[C++20] [Modules] [Itanium ABI] Generate the vtable in the module unit of dynamic classes (#75912)
Close https://github.com/llvm/llvm-project/issues/70585 and reflect
https://github.com/itanium-cxx-abi/cxx-abi/issues/170.
The significant change of the patch is: for dynamic classes attached to
module units, we generate the vtable to the attached module units
directly and the key functions for such classes is meaningless.