Rollup merge of #110671 - compiler-errors:polarity, r=lcnr

Consider polarity in new solver

It's kinda ugly to have a polarity check in all of the builtin impls -- I guess I could consider the polarity at the top of assemble-builtin but that would require adding a polarity fn to `GoalKind`...

🤷 putting this up just so i dont forget, since it's needed to bootstrap core during coherence (this alone does not allow core to bootstrap though, additional work is needed!)

r? ``@lcnr``

compiler/rustc_middle/src/ty/mod.rs

@@ -861,6 +861,11 @@ impl<'tcx> PolyTraitPredicate<'tcx> {
     pub fn is_const_if_const(self) -> bool {
         self.skip_binder().is_const_if_const()
     }
+
+    #[inline]
+    pub fn polarity(self) -> ImplPolarity {
+        self.skip_binder().polarity
+    }
 }
 
 /// `A: B`