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/cloner.go

@@ -169,7 +169,7 @@ func gen(buf *bytes.Buffer, it *codegen.ImportTracker, typ *types.Named) {
 			writef("}")
 		case *types.Map:
 			elem := ft.Elem()
-			if sliceType, isSlice := elem.(*types.Slice); isSlice {
+			if sliceType, isSlice := elem.Underlying().(*types.Slice); isSlice {
 				n := it.QualifiedName(sliceType.Elem())
 				writef("if dst.%s != nil {", fname)
 				writef("\tdst.%s = map[%s]%s{}", fname, it.QualifiedName(ft.Key()), it.QualifiedName(elem))