Rollup merge of #107004 - compiler-errors:new-solver-new-candidates-2, r=lcnr

Implement some candidates for the new solver (redux)

Based on #106718, so the diff is hard to read without it. See [here](https://github.com/rust-lang/rust/compare/98700cf481bce946bff316b56836cfffd885127b...compiler-errors:rust:new-solver-new-candidates-2) for an easier view until that one lands.

Of note:
* 44af916020fb43c12070125c45b6dee4ec303bbc fixes a bug where we need to make the query response *inside* of a probe, or else we make no inference progress (I think)
* 50daad5acd2f163d03e7ffab942534f09bc36e2e implements `consider_assumption` for traits and predicates. I'm not sure if using `sup` here is necessary or if `eq` is fine.
* We decided that all of the `instantiate_constituent_tys_for_*` functions are verbose but ok, since they need to be exhaustive and the logic between each of them is not similar enough, right?

r? ``@lcnr``

compiler/rustc_middle/src/ty/mod.rs

@@ -2357,6 +2357,11 @@ impl<'tcx> TyCtxt<'tcx> {
         self.trait_def(trait_def_id).has_auto_impl
     }
 
+    /// Returns `true` if this is a trait alias.
+    pub fn trait_is_alias(self, trait_def_id: DefId) -> bool {
+        self.def_kind(trait_def_id) == DefKind::TraitAlias
+    }
+
     pub fn trait_is_coinductive(self, trait_def_id: DefId) -> bool {
         self.trait_is_auto(trait_def_id) || self.lang_items().sized_trait() == Some(trait_def_id)
     }