[flang] Add evaluate::GetDerivedTypeSpec(DynamicType)

It gets the semantics::DerivedTypeSpec of a DynamicType if it has one.
Make use of it where it simplifies the code.

Original-commit: flang-compiler/f18@9ad12e7c131459a8beab2b9ffa204966a85f405a
Reviewed-on: https://github.com/flang-compiler/f18/pull/872