cmd/cloner, cmd/viewer: handle named map/slice types with Clone/View methods

The cloner and viewer code generators didn't handle named types
with basic underlying types (map/slice) that have their own Clone
or View methods. For example, a type like:

    type Map map[string]any
    func (m Map) Clone() Map { ... }
    func (m Map) View() MapView { ... }

When used as a struct field, the cloner would descend into the
underlying map[string]any and fail because it can't clone the any
(interface{}) value type. Similarly, the viewer would try to create
a MapFnOf view and fail.

Fix the cloner to check for a Clone method on the named type
before falling through to the underlying type handling.

Fix the viewer to check for a View method on named map/slice types,
so the type author can provide a purpose-built safe view that
doesn't leak raw any values. Named map/slice types without a View
method fall through to normal handling, which correctly rejects
types like map[string]any as unsupported.

Updates tailscale/corp#39502 (needed by tailscale/corp#39594)

Change-Id: Iaef0192a221e02b4b8e409c99ef8398090327744
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/cloner/cloner_test.go

@@ -154,6 +154,34 @@ func TestMapWithPointers(t *testing.T) {
 	}
 }
 
+func TestNamedMapContainer(t *testing.T) {
+	orig := &clonerex.NamedMapContainer{
+		Attrs: clonerex.NamedMap{
+			"str":  "hello",
+			"num":  int64(42),
+			"bool": true,
+		},
+	}
+
+	cloned := orig.Clone()
+	if !reflect.DeepEqual(orig, cloned) {
+		t.Errorf("Clone() = %v, want %v", cloned, orig)
+	}
+
+	// Mutate the cloned map to verify no aliasing.
+	cloned.Attrs["str"] = "modified"
+	if orig.Attrs["str"] == "modified" {
+		t.Errorf("Clone() aliased memory in Attrs: original was modified")
+	}
+
+	// Verify nil handling.
+	nilContainer := &clonerex.NamedMapContainer{}
+	nilClone := nilContainer.Clone()
+	if !reflect.DeepEqual(nilContainer, nilClone) {
+		t.Errorf("Clone() of nil Attrs = %v, want %v", nilClone, nilContainer)
+	}
+}
+
 func TestDeeplyNestedMap(t *testing.T) {
 	num := 123
 	orig := &clonerex.DeeplyNestedMap{