[flang] add nested DEC STRUCTURE in DerivedTypeDetails component names

Currently, when a (legacy) DEC structure contained other DEC structure
declarations, the related component names were not added to the
containing DerivedTypeDetails component_names. This lead to bugs in
later phase when visiting the components (like in when lowering the
type to FIR/MLIR).

When an EntityDecl is visited and the scope is a DEC structure, add
the entity to the component names of this DEC structure.

Differential Revision: https://reviews.llvm.org/D154216