Remove return_type_id from Function. (#4051)

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.

toolchain/sem_ir/function.cpp

@@ -38,4 +38,11 @@ auto GetCalleeFunction(const File& sem_ir, InstId callee_id) -> CalleeFunction {
   return result;
 }
 
+auto Function::declared_return_type(const File& file) const -> TypeId {
+  if (!return_storage_id.is_valid()) {
+    return TypeId::Invalid;
+  }
+  return file.insts().Get(return_storage_id).type_id();
+}
+
 }  // namespace Carbon::SemIR