Key path subscript formatting loses parameter name whitespace

While subscripts with parameter names preserve whitespace, _e.g._ `foo[id: 0]`, the key path version trims it, _e.g._ `\Foo.[id:0]`:

```sh
$ swift-format -v
509.0.0
$ echo "foo[id: 0]" | swift-format
foo[id: 0]
$ echo "\Foo.[id: 0]" | swift-format
\Foo.[id:0]
```