Rollup merge of #143764 - dianne:primary-binding-drop-order, r=Nadrieril,traviscross

lower pattern bindings in the order they're written and base drop order on primary bindings' order

To fix rust-lang/rust#142163, this PR does two things:
- Makes match arms base their drop order on the first sub-branch instead of the last sub-branch. Together with the second change, this makes bindings' drop order correspond to the relative order of when each binding first appears (i.e. the order of the "primary" bindings).
- Lowers pattern bindings in the order they're written (still treating the right-hand side of a ``@`` as coming before the binding on the left). In each sub-branch of a match arm, this is the order that would be obtained if the or-alternatives chosen in that sub-branch were inlined into the arm's pattern. This both affects drop order (making bindings in or-patterns not be dropped first) and fixes the issue in [this test](https://github.com/rust-lang/rust/blob/2a023bf80a6fbd6a06d5460a34eb247b986286ed/tests/ui/pattern/bindings-after-at/bind-by-copy-or-pat.rs) from rust-lang/rust#121716.

My approach to the second point is admittedly a bit trickier than may be necessary. To avoid passing around a counter when building `FlatPat`s, I've instead added just enough information to recover the original structure of the pattern's bindings from a `MatchTreeSubBranch`'s path through the `Candidate` tree. Some alternatives:
- We could use a counter, then sort bindings by their ordinals when making `MatchTreeSubBranch`es.
- I'd like to experiment with always merging sub-candidates and removing `test_remaining_match_pairs_after_or`; that would require lowering bindings and guards in a different way. That makes it a bigger change too, though, so I figure it might be simplest to start here.
- For a very big change, we could track which or-alternatives succeed at runtime to base drop order on the binding order in the particular alternatives matched.

This is a breaking change. It will need a crater run, language team sign-off, and likely updates to the Reference.

This will conflict with rust-lang/rust#143376 and probably also rust-lang/rust#143028, so they shouldn't be merged at the same time.

r? `@matthewjasper` or `@Nadrieril`

Author: Zalathar