[flang] Adapt PolymorphicOpConversion to run on all top level ops (#90597)

We might use polymorphic ops in top-level operations other than
functions some time in the future. We need to ensure that these
operations can be lowered.

See RFC:

https://discourse.llvm.org/t/rfc-add-an-interface-for-top-level-container-operations

Some of the changes are from moving declaration and definition of the
constructor function into tablegen (as requested in code review when
altering another pass).