fix: Prevent infinite loops when parsing items from cfg_select! arms

[ytmimi]

Fixes #7088

Before when we failed to parse an item we'd hit an infinite loop because we wouldn't advance the parser. Now we make sure to advance the parser before starting the next loop iteration.

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

[jieyouxu]

The change itself seems reasonable. I'm unsure on the formatting of

cfg_select! {
    _ => {
        {}
        {}
    }
}

to

cfg_select! { _ => {{} {}}}

That seems... a bit curious? Oh I see, that's the source formatting.

@rustbot author

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[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[ytmimi]

Oh I see what's going on. The stable tests are failing because cfg_select! isn't getting formatted at all on stable because of our is_nightly_channel! gate. When comparing the source file with the target file it seems like cfg_select! { _ => {{} {}}} is getting formatted, but its not, and the diff we're seeing is just the diff between the source and target files. At least I'm pretty sure that's what's going on.

-cfg_select! {
-    _ => {
-        {}
-        {}
-    }
-}
+cfg_select! { _ => {{} {}}}

[jieyouxu]

Ah right, yeah that'd make sense

[jieyouxu]

Thanks

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