dillon1000/react

Make LegacyHidden match semantics of old fork (#18998)

Facebook currently relies on being able to hydrate hidden HTML. So
skipping those trees is a regression.

We don't have a proper solution for this in the new API yet. So I'm
reverting it to match the old behavior.

Now the server renderer will treat LegacyHidden the same as a fragment,
with no other special behavior. We can only get away with this because
we assume that every instance of LegacyHidden is accompanied by a host
component wrapper. In the hidden mode, the host component is given a
`hidden` attribute, which ensures that the initial HTML is not visible.
To support the use of LegacyHidden as a true fragment, without an extra
DOM node, we will have to hide the initial HTML in some other way.