cmd/cloner: deep-clone pointer elements in map-of-slice values

The cloner's codegen for map[K][]*V fields was doing a shallow
append (copying pointer values) instead of cloning each element.
This meant that cloned structs aliased the original's pointed-to
values through the map's slice entries.

Mirror the existing standalone-slice logic that checks
ContainsPointers(sliceType.Elem()) and generates per-element
cloning for pointer, interface, and struct types.

Regenerate net/dns and tailcfg which both had affected
map[...][]*dnstype.Resolver fields.

Fixes #19284

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

cmd/cloner/clonerex/clonerex.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:generate go run tailscale.com/cmd/cloner  -clonefunc=true -type SliceContainer,InterfaceContainer,MapWithPointers,DeeplyNestedMap,NamedMapContainer
+//go:generate go run tailscale.com/cmd/cloner  -clonefunc=true -type SliceContainer,InterfaceContainer,MapWithPointers,DeeplyNestedMap,NamedMapContainer,MapSlicePointerContainer
 
 // Package clonerex is an example package for the cloner tool.
 package clonerex
@@ -60,6 +60,13 @@ type NamedMapContainer struct {
 	Attrs NamedMap
 }
 
+// MapSlicePointerContainer has a map whose values are slices of pointers.
+// This tests that the cloner deep-clones the pointer elements in the slice,
+// not just the slice itself (which would leave aliased pointers).
+type MapSlicePointerContainer struct {
+	Routes map[string][]*SliceContainer
+}
+
 // DeeplyNestedMap tests arbitrary depth of map nesting (3+ levels)
 type DeeplyNestedMap struct {
 	ThreeLevels map[string]map[string]map[string]int