[flang] Introduce BaseBoxType

Introduce a BaseBoxType to be used by BoxType and
the a new ClassType that is introduced in a follow up patch.

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

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D134956