Auto merge of #101241 - camsteffen:refactor-binding-annotations, r=cjgillot

`BindingAnnotation` refactor

* `ast::BindingMode` is deleted and replaced with `hir::BindingAnnotation` (which is moved to `ast`)
* `BindingAnnotation` is changed from an enum to a tuple struct e.g. `BindingAnnotation(ByRef::No, Mutability::Mut)`
* Associated constants added for convenience `BindingAnnotation::{NONE, REF, MUT, REF_MUT}`

One goal is to make it more clear that `BindingAnnotation` merely represents syntax `ref mut` and not the actual binding mode. This was especially confusing since we had `ast::BindingMode`->`hir::BindingAnnotation`->`thir::BindingMode`.

I wish there were more symmetry between `ByRef` and `Mutability` (variant) naming (maybe `Mutable::Yes`?), and I also don't love how long the name `BindingAnnotation` is, but this seems like the best compromise. Ideas welcome.

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -367,7 +367,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
                 if let Some(Node::Pat(pat)) = self.infcx.tcx.hir().find(upvar_hir_id)
                     && let hir::PatKind::Binding(
-                        hir::BindingAnnotation::Unannotated,
+                        hir::BindingAnnotation::NONE,
                         _,
                         upvar_ident,
                         _,