Prevent query block from looping in classic themes

[draganescu]

What?

Closes #43198

Why?

In classic themes templates depend on have_posts() to return false to exit the loop. The change added by #40656
uses the global WP_Query to figure out global query when inherit is on. This in turn loops over the global posts
object and resets it once it reaches the end. Thus when rendering posts and meeting one post using the query loop block the
upper most have_posts is never false (because the global posts always get reset).

How?

An easy and quick fix is to clone the global WP_Query. This allows the block to render the loop without resetting the
global posts. This way the block renders the correct loop but does not interfere with the global loop.

Testing Instructions

0. Using this Gutenberg and this PR
1. Activate a classic theme
2. Create a post and add a query block in it
3. Set up the block by clicking:
   a) choose
   b) then choose again in the modal
4. In the block inspector toggle on "Inherit query from template"
5. Publish the post
6. Visit the post
7. There should be no endless loop, the page should render all right.

Screenshots or screencast

N/A

[martinkrcho]

Cloning the WP_Query class should be fine as long as none of the properties that are objects are modified (read more on this topic in Don't clone your php objects, DeepCopy them).

Otherwise the global query would be working with altered objects later on. That doesn't seem to be the case here since the query is used to loop through a list of posts and render them. Even though I would recommend running some tests where the query parameters include meta and taxonomy queries.

However, I think the post data should be reset using wp_reset_postdata further down in code even when the global query is used.

[martinkrcho]

Suggested changes look good to me. As long as the block only loops through the posts (rather than changing the query properties that are objects), cloning is sufficient here.

[oandregal]

Removing the "Backport to Gutenberg RC" as this bug seems to have been introduced in April, not during the 13.9 release cycle. It can go in the next Gutenberg release.

[draganescu]

Merged w/ failing react native tests as to me they seemed unrelated 🙏🏻

[gziolo]

Cherry-picked with 3c19b1b for WordPress 6.0.2 release.