Rollup merge of #119817 - compiler-errors:normalize-opaques, r=lcnr

Remove special-casing around `AliasKind::Opaque` when structurally resolving in new solver

This fixes a few inconsistencies around where we don't eagerly resolve opaques to their (locally-defined) hidden types in the new solver. It essentially allows this code to work:
```rust
fn main() {
    type Tait = impl Sized;
    struct S {
        i: i32,
    }
    let x: Tait = S { i: 0 };
    println!("{}", x.i);
}
```

Since `Tait` is defined in `main`, we are able to poke through the type of `x` with deref.

r? lcnr

compiler/rustc_trait_selection/src/traits/structural_normalize.rs

@@ -22,8 +22,7 @@ impl<'tcx> StructurallyNormalizeExt<'tcx> for At<'_, 'tcx> {
         assert!(!ty.is_ty_var(), "should have resolved vars before calling");
 
         if self.infcx.next_trait_solver() {
-            // FIXME(-Znext-solver): Should we resolve opaques here?
-            let ty::Alias(ty::Projection | ty::Inherent | ty::Weak, _) = *ty.kind() else {
+            let ty::Alias(..) = *ty.kind() else {
                 return Ok(ty);
             };