[flang] Rework how DeclTypeSpecs are stored

Now DeclTypeSpecs are stored in the scope so that they remain available
as long as the scope exists. DeclTypeSpecs for intrinsic types are
stored in the global scope; those for derived types are in the current
scope. They can contains type parameter values so they can't be reused.

Add `Semantics::MakeIntrinsicTypeSpec` to simplify creating
DeclTypeSpecs for intrinsic types.

Replace `std::optional<DeclTypeSpec>` with `DeclTypeSpec *` as they do
not need to be copied around.

Also fix a small bug with writing `class(t(...))` to the module file --
the type parameters were missing.

Original-commit: flang-compiler/f18@e4744418fc1cdc9728bba4b9ecde51cb5bdd2efa
Reviewed-on: https://github.com/flang-compiler/f18/pull/247
Tree-same-pre-rewrite: false