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>

cmd/cloner/cloner.go

@@ -115,7 +115,7 @@ func gen(buf *bytes.Buffer, it *codegen.ImportTracker, typ *types.Named) {
 		if !codegen.ContainsPointers(ft) || codegen.HasNoClone(t.Tag(i)) {
 			continue
 		}
-		if named, _ := ft.(*types.Named); named != nil {
+		if named, _ := codegen.NamedTypeOf(ft); named != nil {
 			if codegen.IsViewType(ft) {
 				writef("dst.%s = src.%s", fname, fname)
 				continue
@@ -161,7 +161,7 @@ func gen(buf *bytes.Buffer, it *codegen.ImportTracker, typ *types.Named) {
 		case *types.Pointer:
 			base := ft.Elem()
 			hasPtrs := codegen.ContainsPointers(base)
-			if named, _ := base.(*types.Named); named != nil && hasPtrs {
+			if named, _ := codegen.NamedTypeOf(base); named != nil && hasPtrs {
 				writef("dst.%s = src.%s.Clone()", fname, fname)
 				continue
 			}