[flang] Refactor DeclTypeSpec and DerivedTypeSpec classes.

Make DerivedTypeSpec a simple wrapper around the name of the type.
Leave out type parameter values until we are ready to resolve them
as well.

Change DeclTypeSpec to be an old-fashioned union with an enum to
indicate what is in it. std::variant doesn't work well here because
we wanted the enum visible in the class' API and there is not a
1-to-1 mapping between enumerators and data stored.

Original-commit: flang-compiler/f18@03bdeef790375ea7dbac6abf1118ec743c55a1b6
Reviewed-on: https://github.com/flang-compiler/f18/pull/109
Tree-same-pre-rewrite: false