[flang][CodeGen][NFC] Reduce boilerplatre for ExternalNameConversion (#94474)

Use tablegen to generate the pass constructor.

I removed the duplicated pass option handling. I don't understand why
the manual instantiation of the pass needs its own duplicate of the pass
options in the (automatically generated) base class (even with the
option to ignore the pass options in the base class).

This pass doesn't need changes to support other top level operations.