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/viewer/tests/tests.go

@@ -204,13 +204,27 @@ type StructWithContainers struct {
 }
 
 type (
-	StructWithPtrsAlias    = StructWithPtrs
-	StructWithoutPtrsAlias = StructWithoutPtrs
+	StructWithPtrsAlias        = StructWithPtrs
+	StructWithoutPtrsAlias     = StructWithoutPtrs
+	StructWithPtrsAliasView    = StructWithPtrsView
+	StructWithoutPtrsAliasView = StructWithoutPtrsView
 )
 
 type StructWithTypeAliasFields struct {
 	WithPtr    StructWithPtrsAlias
 	WithoutPtr StructWithoutPtrsAlias
+
+	WithPtrByPtr    *StructWithPtrsAlias
+	WithoutPtrByPtr *StructWithoutPtrsAlias
+
+	SliceWithPtrs    []*StructWithPtrsAlias
+	SliceWithoutPtrs []*StructWithoutPtrsAlias
+
+	MapWithPtrs    map[string]*StructWithPtrsAlias
+	MapWithoutPtrs map[string]*StructWithoutPtrsAlias
+
+	MapOfSlicesWithPtrs    map[string][]*StructWithPtrsAlias
+	MapOfSlicesWithoutPtrs map[string][]*StructWithoutPtrsAlias
 }
 
 type integer = constraints.Integer