[flang][NFC] use tablegen to create StackArrays constructor (#90038)

Stack arrays needs to stay running only on func.func because it needs to
know which block terminators can end the function (rather than just
branch between unstructured control flow). A similar concept does not
exist at the more abstract level of "any top level mlir operation".

For example, it currently looks for func::ReturnOp and
fir::UnreachableOp as points when execution can end. If this were to be
run on omp.declare_reduction, it would also need to understand
omp.YieldOp (perhaps only when omp.declare_reduction is the parent).
There isn't a generic concept in MLIR for this.