cmd/cloner, cmd/viewer, util/codegen: add support for aliases of cloneable types

We have several checked type assertions to *types.Named in both cmd/cloner and cmd/viewer.
As Go 1.23 updates the go/types package to produce Alias type nodes for type aliases,
these type assertions no longer work as expected unless the new behavior is disabled
with gotypesalias=0.

In this PR, we add codegen.NamedTypeOf(t types.Type), which functions like t.(*types.Named)
but also unrolls type aliases. We then use it in place of type assertions in the cmd/cloner and
cmd/viewer packages where appropriate.

We also update type switches to include *types.Alias alongside *types.Named in relevant cases,
remove *types.Struct cases when switching on types.Type.Underlying and update the tests
with more cases where type aliases can be used.

Updates #13224
Updates #12912

Signed-off-by: Nick Khyl <nickk@tailscale.com>

util/codegen/codegen.go

@@ -382,3 +382,12 @@ func LookupMethod(t types.Type, name string) *types.Func {
 	}
 	return nil
 }
+
+// NamedTypeOf is like t.(*types.Named), but also works with type aliases.
+func NamedTypeOf(t types.Type) (named *types.Named, ok bool) {
+	if a, ok := t.(*types.Alias); ok {
+		return NamedTypeOf(types.Unalias(a))
+	}
+	named, ok = t.(*types.Named)
+	return
+}