[flang] Fix output type when emboxing intrinsic type to unlimited polymorphic box

When emboxing an intrinsic type to a polymorphic descriptor, directly set its
type to `fir.class<none>`.

`fir.class<i32>` is not a real type used anywhere in lowering so make it right directly
avoid unnecessary convert op to `fir.class<none>`. Also `fir.class<i32>` would not be
recognized as unlimited polymorphic.

Reviewed By: jeanPerier, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D143888