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>

net/dns/dns_clone.go

@@ -6,7 +6,6 @@
 package dns
 
 import (
-	"maps"
 	"net/netip"
 
 	"tailscale.com/types/dnstype"
@@ -45,7 +44,7 @@ func (src *Config) Clone() *Config {
 			dst.Hosts[k] = append([]netip.Addr{}, src.Hosts[k]...)
 		}
 	}
-	dst.SubdomainHosts = maps.Clone(src.SubdomainHosts)
+	dst.SubdomainHosts = src.SubdomainHosts.Clone()
 	return dst
 }