util/codegen: treat unique.Handle as an opaque value type

It doesn't need a Clone method, like a time.Time, etc.

And then, because Go 1.23+ uses unique.Handle internally for
the netip package types, we can remove those special cases.

Updates #14058 (pulled out from that PR)
Updates tailscale/corp#24485

Change-Id: Iac3548a9417ccda5987f98e0305745a6e178b375
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/codegen/codegen_test.go

@@ -10,6 +10,8 @@ import (
 	"strings"
 	"sync"
 	"testing"
+	"time"
+	"unique"
 	"unsafe"
 
 	"golang.org/x/exp/constraints"
@@ -84,6 +86,16 @@ type PointerUnionParam[T netip.Prefix | BasicType | IntPtr] struct {
 	V T
 }
 
+type StructWithUniqueHandle struct{ _ unique.Handle[[32]byte] }
+
+type StructWithTime struct{ _ time.Time }
+
+type StructWithNetipTypes struct {
+	_ netip.Addr
+	_ netip.AddrPort
+	_ netip.Prefix
+}
+
 type Interface interface {
 	Method()
 }
@@ -161,6 +173,18 @@ func TestGenericContainsPointers(t *testing.T) {
 			typ:         "PointerUnionParam",
 			wantPointer: true,
 		},
+		{
+			typ:         "StructWithUniqueHandle",
+			wantPointer: false,
+		},
+		{
+			typ:         "StructWithTime",
+			wantPointer: false,
+		},
+		{
+			typ:         "StructWithNetipTypes",
+			wantPointer: false,
+		},
 	}
 
 	for _, tt := range tests {