Instead of redundantly storing both the `return_type_id` and
`return_storage_id`, where the declared return type is just the type of
the return storage, store only the `return_storage_id`.
Add a convenience property to get the declared return type of the
function.
In addition to avoiding storing redundant information, this is a
preparatory step for an upcoming change for generics support that will
make it more expensive and awkward to store `TypeId`s in places other
than the type of an instruction.
Richard Smith
e3c15edb92