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/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
+//go:generate go run tailscale.com/cmd/cloner  -clonefunc=true -type SliceContainer,InterfaceContainer,MapWithPointers,DeeplyNestedMap,NamedMapContainer
 
 // Package clonerex is an example package for the cloner tool.
 package clonerex
@@ -39,6 +39,27 @@ type MapWithPointers struct {
 	CloneInterface  map[string]Cloneable
 }
 
+// NamedMap is a named map type with its own Clone method.
+// This tests that the cloner uses the type's Clone method
+// rather than trying to descend into the map's value type.
+type NamedMap map[string]any
+
+func (m NamedMap) Clone() NamedMap {
+	if m == nil {
+		return nil
+	}
+	m2 := make(NamedMap, len(m))
+	for k, v := range m {
+		m2[k] = v
+	}
+	return m2
+}
+
+// NamedMapContainer has a field whose type is a named map with a Clone method.
+type NamedMapContainer struct {
+	Attrs NamedMap
+}
+
 // DeeplyNestedMap tests arbitrary depth of map nesting (3+ levels)
 type DeeplyNestedMap struct {
 	ThreeLevels map[string]map[string]map[string]int