[flang] Use fir.type_desc in nullify

Do not look for the global early in nullify codegen. The type descriptor
can be emitted later and it would raise an error as it could not be found.
Use `fir.type_desc` instead so it delays the type descriptor lookup until
evrything is emitted.

https://github.com/llvm/llvm-project/issues/63775

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D154982