Rollup merge of #64111 - Centril:ast-only-patkind-or, r=petrochenkov

or-patterns: Uniformly use `PatKind::Or` in AST & Fix/Cleanup resolve

Following up on work in https://github.com/rust-lang/rust/pull/63693 and https://github.com/rust-lang/rust/pull/61708, in this PR we:

- Uniformly use `PatKind::Or(...)` in AST:

   - Change `ast::Arm.pats: Vec<P<Pat>>` => `ast::Arm.pat: P<Pat>`

   - Change `ast::ExprKind::Let.0: Vec<P<Pat>>` => `ast::ExprKind::Let.0: P<Pat>`

- Adjust `librustc_resolve/late.rs` to correctly handle or-patterns at any level of nesting as a result.

  In particular, the already-bound check which rejects e.g. `let (a, a);` now accounts for or-patterns. The consistency checking (ensures no missing bindings and binding mode consistency) also now accounts for or-patterns. In the process, a bug was found in the current compiler which allowed:

   ```rust
   enum E<T> { A(T, T), B(T) }
   use E::*;
   fn foo() {
       match A(0, 1) {
           B(mut a) | A(mut a, mut a) => {}
       }
   }
   ```

   The new algorithms took a few iterations to get right. I tried several clever schemes but ultimately a version based on a stack of hashsets and recording product/sum contexts was chosen since it is more clearly correct.

- Clean up `librustc_resolve/late.rs` by, among other things, using a new `with_rib` function to better ensure stack dicipline.

- Do not push the change in AST to HIR for now to avoid doing too much in this PR. To cope with  this, we introduce a temporary hack in `rustc::hir::lowering` (clearly marked in the diff).

cc https://github.com/rust-lang/rust/issues/54883
cc @dlrobertson @matthewjasper
r? @petrochenkov

src/libsyntax_ext/format.rs

@@ -716,7 +716,7 @@ impl<'a, 'b> Context<'a, 'b> {
         // But the nested match expression is proved to perform not as well
         // as series of let's; the first approach does.
         let pat = self.ecx.pat_tuple(self.fmtsp, pats);
-        let arm = self.ecx.arm(self.fmtsp, vec![pat], args_array);
+        let arm = self.ecx.arm(self.fmtsp, pat, args_array);
         let head = self.ecx.expr(self.fmtsp, ast::ExprKind::Tup(heads));
         let result = self.ecx.expr_match(self.fmtsp, head, vec![arm]);