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- remove outdated fixme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/coherence.rs

@@ -95,8 +95,11 @@ pub fn overlapping_impls(
         return None;
     }
 
-    let infcx =
-        tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).intercrate().build();
+    let infcx = tcx
+        .infer_ctxt()
+        .with_opaque_type_inference(DefiningAnchor::Bubble)
+        .intercrate(true)
+        .build();
     let selcx = &mut SelectionContext::new(&infcx);
     let overlaps =
         overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).is_some();
@@ -107,8 +110,11 @@ pub fn overlapping_impls(
     // In the case where we detect an error, run the check again, but
     // this time tracking intercrate ambiguity causes for better
     // diagnostics. (These take time and can lead to false errors.)
-    let infcx =
-        tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).intercrate().build();
+    let infcx = tcx
+        .infer_ctxt()
+        .with_opaque_type_inference(DefiningAnchor::Bubble)
+        .intercrate(true)
+        .build();
     let selcx = &mut SelectionContext::new(&infcx);
     selcx.enable_tracking_intercrate_ambiguity_causes();
     Some(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).unwrap())