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_clone.go

@@ -159,9 +159,26 @@ var _DeeplyNestedMapCloneNeedsRegeneration = DeeplyNestedMap(struct {
 	FourLevels  map[string]map[string]map[string]map[string]*SliceContainer
 }{})
 
+// Clone makes a deep copy of NamedMapContainer.
+// The result aliases no memory with the original.
+func (src *NamedMapContainer) Clone() *NamedMapContainer {
+	if src == nil {
+		return nil
+	}
+	dst := new(NamedMapContainer)
+	*dst = *src
+	dst.Attrs = src.Attrs.Clone()
+	return dst
+}
+
+// A compilation failure here means this code must be regenerated, with the command at the top of this file.
+var _NamedMapContainerCloneNeedsRegeneration = NamedMapContainer(struct {
+	Attrs NamedMap
+}{})
+
 // Clone duplicates src into dst and reports whether it succeeded.
 // To succeed, <src, dst> must be of types <*T, *T> or <*T, **T>,
-// where T is one of SliceContainer,InterfaceContainer,MapWithPointers,DeeplyNestedMap.
+// where T is one of SliceContainer,InterfaceContainer,MapWithPointers,DeeplyNestedMap,NamedMapContainer.
 func Clone(dst, src any) bool {
 	switch src := src.(type) {
 	case *SliceContainer:
@@ -200,6 +217,15 @@ func Clone(dst, src any) bool {
 			*dst = src.Clone()
 			return true
 		}
+	case *NamedMapContainer:
+		switch dst := dst.(type) {
+		case *NamedMapContainer:
+			*dst = *src.Clone()
+			return true
+		case **NamedMapContainer:
+			*dst = src.Clone()
+			return true
+		}
 	}
 	return false
 }