Auto merge of #104986 - compiler-errors:opaques, r=oli-obk

Combine `ty::Projection` and `ty::Opaque` into `ty::Alias`

Implements https://github.com/rust-lang/types-team/issues/79.

This PR consolidates `ty::Projection` and `ty::Opaque` into a single `ty::Alias`, with an `AliasKind` and `AliasTy` type (renamed from `ty::ProjectionTy`, which is the inner data of `ty::Projection`) defined as so:

```
enum AliasKind {
  Projection,
  Opaque,
}

struct AliasTy<'tcx> {
  def_id: DefId,
  substs: SubstsRef<'tcx>,
}
```

Since we don't have access to `TyCtxt` in type flags computation, and because repeatedly calling `DefKind` on the def-id is expensive, these two types are distinguished with `ty::AliasKind`, conveniently glob-imported into `ty::{Projection, Opaque}`. For example:

```diff
  match ty.kind() {
-   ty::Opaque(..) =>
+   ty::Alias(ty::Opaque, ..) => {}
    _ => {}
  }
```

This PR also consolidates match arms that treated `ty::Opaque` and `ty::Projection` identically.

r? `@ghost`

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -697,8 +697,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     .map_bound(|p| p.predicates),
                 None,
             ),
-            ty::Opaque(did, substs) => {
-                find_fn_kind_from_did(tcx.bound_explicit_item_bounds(*did), Some(*substs))
+            ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs }) => {
+                find_fn_kind_from_did(tcx.bound_explicit_item_bounds(*def_id), Some(*substs))
             }
             ty::Closure(_, substs) => match substs.as_closure().kind() {
                 ty::ClosureKind::Fn => Some(hir::Mutability::Not),

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -504,7 +504,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;
 
         let mut output_ty = self.regioncx.universal_regions().unnormalized_output_ty;
-        if let ty::Opaque(def_id, _) = *output_ty.kind() {
+        if let ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs: _ }) = *output_ty.kind() {
             output_ty = self.infcx.tcx.type_of(def_id)
         };