Rollup merge of #121716 - Nadrieril:simple-binding-order, r=matthewjasper

match lowering: Lower bindings in a predictable order

After the recent refactorings, we can now lower bindings in a truly predictable order. The order in rust-lang/rust#120214 was an improvement but not very clear. With this PR, we lower bindings from left to right, with the special case that `x @ pat` is traversed as `pat @ x` (i.e. `x` is lowered after any bindings in `pat`).

This description only applies in the absence of or-patterns. Or-patterns make everything complicated, because the binding place depends on the subpattern. Until I have a better idea I leave them to be handled in whatever weird order arises from today's code.

r? `@matthewjasper`

Author: Nadrieril