We leak inference variables from snapshots

[lcnr]

"snapshot vec: index out of bounds" errors happen if we leak inference variables from a probe/in the error of a commit_if_ok.

We create an inference variables inside of the snapshot, revert the snapshot, removing the infer var again, and then try to look it up in the InferCtxt which then causes this ICE. This is already an existing issue and a bunch of diagnostics are really brittle right now, ICEing if you look at them the wrong way. I'Ve tried to fix this in #122189 but didn't end up getting it over the finish line.

Originally posted by @lcnr in #142488

Given that this is such a frequent issue and makes working on diagnostics a more unpleasant than it should be, we should actually do a proper fix here.

[lcnr]

closing as duplicate of #122188