Rollup merge of #102945 - compiler-errors:placeholder-region-outlives, r=lcnr

Do not register placeholder `RegionOutlives` obligations when `considering_regions` is false

**NOTE:** I'm kinda just putting this up for discussion. I'm not certain this is correct...?

This was introduced in [`608625d`](608625dae9 (diff-6e54b18681342ec725d75591dbf384ad08cd73df29db00485fe51b4e90f76ff7R361)).

Interestingly, we only check `data.has_placeholders()` for `RegionOutlives`, and not for `TypeOutlives`... why? For the record, that different treatment between `RegionOutlives` and `TypeOutlives` is why the fix "The compiling succeeds when all `'a : 'b` are replaced with `&'a () : 'b`" in #100689 _"works"_, but it seems like an implementation detail considering this.

Also, why do we care about placeholder regions being registered if `considering_regions` is false? It doesn't seem to affect any UI tests, for example.

r? `@lcnr`

Fixes #102899
Fixes #100689

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -355,7 +355,7 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 }
 
                 ty::PredicateKind::RegionOutlives(data) => {
-                    if infcx.considering_regions || data.has_placeholders() {
+                    if infcx.considering_regions {
                         infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
                     }