cmd/cloner: handle named slices as map values (#20387)

Previously cloner only handled literal slices for values, like
`map[string][]int`. This adds support for named types with an underlying
type of slice, like `map[string]IntSlice` with `type IntSlice []int`.

Updates tailscale/corp#44077

Signed-off-by: Andrew Lytvynov <awly@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,NamedMapContainer,MapSlicePointerContainer
+//go:generate go run tailscale.com/cmd/cloner  -clonefunc=true -type SliceContainer,InterfaceContainer,MapWithPointers,DeeplyNestedMap,NamedMapContainer,MapSlicePointerContainer,MapWithNamedSliceValues
 
 // Package clonerex is an example package for the cloner tool.
 package clonerex
@@ -72,3 +72,12 @@ type DeeplyNestedMap struct {
 	ThreeLevels map[string]map[string]map[string]int
 	FourLevels  map[string]map[string]map[string]map[string]*SliceContainer
 }
+
+// MapWithNamedSliceValues has a map with a named slice type for values. This
+// tests that the generator treats these values like any other slice and not a
+// struct.
+type MapWithNamedSliceValues struct {
+	M map[string]NamedSlice
+}
+
+type NamedSlice []string