[flang] Introduce fir.class type

Introduce a new ClassType for polymorphic
entities. A fir.class type is similar to a fir.box type in
many ways and is also base on the BaseBoxType.

This patch is part of the implementation of the poltymorphic
entities.
https://github.com/llvm/llvm-project/blob/main/flang/docs/PolymorphicEntities.md

Depends on D134956

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D134957