Replace visit_waiters with waiters_of

[Zoxc]

This replaces visit_waiters which uses closures to visit waiters with waiters_of which returns a list of waiters. This makes the control flow of their users a bit clearer.

Additionally waiters_of includes non-query waiters unlike visit_waiters. This means that connected_to_root now considers resumable waiters from the compiler root as being connected to root, while previously only non-resumable waiters counted. This can result in some additional entry points being found. Similarly in the loop detecting entry points we now consider queries in the cycle with direct resumable waiters from the compiler root as being entry points.

When looking for entry points we now look at waiters until we found a query to populate the EntryPoint.waiter field instead of stopping when we determined it to be an entry point.

cc @petrochenkov @nnethercote

[rustbot]

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[nnethercote]

This is a smallish change to code that #153185 is proposing to completely rewrite. If that PR is accepted and merged then this one will be moot. So we need to reach a conclusion on that PR before considering this one.

@rustbot blocked

[Zoxc]

That's not how that works.