[libc++abi][NFC] Return const void* from __dynamic_cast (#223737)

`__dynamic_cast` is only called magically by the compiler, so it really
doesn't matter what the exact protoype is as long as it's ABI
compatible. Making the return type a `const void*` simplifies the
implementation a bit though.

GitOrigin-RevId: 3723ac3407bc09d0b822df08f17228eed8fa76a3