cmd/cloner: unify switch cases

And in the process, fix a bug:
The fmt formatting was being applied by writef,
not fmt.Sprintf, thus emitting a MISSING string.
And there's no guarantee that fmt will be imported
in the generated code.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

cmd/cloner/cloner.go

@@ -211,10 +211,8 @@ func gen(buf *bytes.Buffer, imports map[string]struct{}, typ *types.Named, thisP
 				writef("\t}")
 			}
 			writef("}")
-		case *types.Struct:
-			writef(`panic("TODO struct %s")`, fname)
 		default:
-			writef(`panic(fmt.Sprintf("TODO: %T", ft))`)
+			writef(`panic("TODO: %s (%T)")`, fname, ft)
 		}
 	}
 	writef("return dst")