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- remove outdated fixme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_trait_selection/src/traits/mod.rs

@@ -4,7 +4,7 @@
 
 pub mod auto_trait;
 mod chalk_fulfill;
-mod coherence;
+pub(crate) mod coherence;
 pub mod const_evaluatable;
 mod engine;
 pub mod error_reporting;