util/codegen, cmd/cloner, cmd/viewer: update codegen.LookupMethod to support alias type nodes

Go 1.23 updates the go/types package to produce Alias type nodes for type aliases, unless disabled with gotypesalias=0.
This new default behavior breaks codegen.LookupMethod, which uses checked type assertions to types.Named and
types.Interface, as only named types and interfaces have methods.

In this PR, we update codegen.LookupMethod to perform method lookup on the right-hand side of the alias declaration
and clearly switch on the supported type nodes types. We also improve support for various edge cases, such as when an alias
is used as a type parameter constraint, and add tests for the LookupMethod function.

Additionally, we update cmd/viewer/tests to include types with aliases used in type fields and generic type constraints.

Updates #13224
Updates #12912

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

cmd/viewer/tests/tests_clone.go

@@ -441,3 +441,41 @@ var _StructWithContainersCloneNeedsRegeneration = StructWithContainers(struct {
 	CloneableMap              MapContainer[int, *StructWithPtrs]
 	CloneableGenericMap       MapContainer[int, *GenericNoPtrsStruct[int]]
 }{})
+
+// Clone makes a deep copy of StructWithTypeAliasFields.
+// The result aliases no memory with the original.
+func (src *StructWithTypeAliasFields) Clone() *StructWithTypeAliasFields {
+	if src == nil {
+		return nil
+	}
+	dst := new(StructWithTypeAliasFields)
+	*dst = *src
+	panic("TODO: WithPtr (*types.Struct)")
+	return dst
+}
+
+// A compilation failure here means this code must be regenerated, with the command at the top of this file.
+var _StructWithTypeAliasFieldsCloneNeedsRegeneration = StructWithTypeAliasFields(struct {
+	WithPtr    StructWithPtrsAlias
+	WithoutPtr StructWithoutPtrsAlias
+}{})
+
+// Clone makes a deep copy of GenericTypeAliasStruct.
+// The result aliases no memory with the original.
+func (src *GenericTypeAliasStruct[T, T2, V2]) Clone() *GenericTypeAliasStruct[T, T2, V2] {
+	if src == nil {
+		return nil
+	}
+	dst := new(GenericTypeAliasStruct[T, T2, V2])
+	*dst = *src
+	dst.Cloneable = src.Cloneable.Clone()
+	return dst
+}
+
+// A compilation failure here means this code must be regenerated, with the command at the top of this file.
+func _GenericTypeAliasStructCloneNeedsRegeneration[T integer, T2 views.ViewCloner[T2, V2], V2 views.StructView[T2]](GenericTypeAliasStruct[T, T2, V2]) {
+	_GenericTypeAliasStructCloneNeedsRegeneration(struct {
+		NonCloneable T
+		Cloneable    T2
+	}{})
+}