Rollup merge of #33345 - birkenfeld:issue-31754, r=pnkfelix

middle: reset loop labels while visiting closure

This should fix #31754 and follow-up #25343.  Before the latter, the closure was visited twice in the context of the enclosing fn, which made even a single closure with a loop label emit a warning.

With this change, the closure is still visited within the context of the main fn (which is intended, since it is not a separate item) but resets the found loop labels while being visited.

Fixes: #31754
Note: I amended the test file from #25343, but I don't know if the original or amended test are effective, since as far as I could see, compiletest's run-pass tests do not check for zero warnings emitted?

/cc @Manishearth