[flang] zero initialized all saved values without initial values (#67693)

This is not standard but is vastly expected by existing code.

This was implemented by https://reviews.llvm.org/D149877 for simple
scalars, but MLIR lacked a generic way to deal with aggregate types
(arrays and derived type).

Support was recently added in
https://github.com/llvm/llvm-project/pull/65508. Leverage it to zero
initialize all types.