Auto merge of #119233 - Nadrieril:keep-whole-pat-around, r=compiler-errors

Exhaustiveness: keep the original `thir::Pat` around

This PR makes it possible for exhaustiveness to look at the original `thir::Pat`, which I'll need at least for the [`small_gaps`](https://github.com/rust-lang/rust/pull/118879) lint (without that we can't distinguish inclusive and exclusive ranges within exhaustiveness). This PR is almost entirely lifetime-wrangling.