Rollup merge of #109447 - lcnr:coherence, r=compiler-errors

new solver cleanup + implement coherence

the cleanup:
- change `Certainty::unify_and` to consider ambig + overflow to be ambig
- rename `trait_candidate_should_be_dropped_in_favor_of` to `candidate_should_be_dropped_in_favor_of`
- remove outdated fixme

For coherence I mostly just add an ambiguous candidate if the current trait ref is unknowable. I am doing the same for reservation impl where I also just add an ambiguous candidate.

compiler/rustc_infer/src/infer/mod.rs

@@ -585,8 +585,8 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
         self
     }
 
-    pub fn intercrate(mut self) -> Self {
-        self.intercrate = true;
+    pub fn intercrate(mut self, intercrate: bool) -> Self {
+        self.intercrate = intercrate;
         self
     }