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>
d52ae45e9b