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/viewer/viewer.go

@@ -282,6 +282,22 @@ func genView(buf *bytes.Buffer, it *codegen.ImportTracker, typ *types.Named, fie
 			writeTemplateWithComment("valueField", fname)
 			continue
 		}
+		// Named map/slice types whose element type is opaque (e.g. any)
+		// can't be safely wrapped in views.Map/views.Slice because the
+		// accessor would leak the raw element.  If the type provides its
+		// own View() method the author can return a purpose-built safe
+		// view; use it.  Otherwise fall through to the normal handling,
+		// which will reject the type as unsupported.
+		if named, _ := codegen.NamedTypeOf(fieldType); named != nil {
+			switch fieldType.Underlying().(type) {
+			case *types.Map, *types.Slice:
+				if viewType := viewTypeForValueType(fieldType); viewType != nil {
+					args.FieldViewName = it.QualifiedName(viewType)
+					writeTemplateWithComment("viewField", fname)
+					continue
+				}
+			}
+		}
 		switch underlying := fieldType.Underlying().(type) {
 		case *types.Slice:
 			slice := underlying