Auto merge of #118824 - aliemjay:perf-region-cons, r=compiler-errors

use Vec for region constraints instead of BTreeMap

~1% perf gain

Diagnostic regressions need more investigation.

r? `@ghost`

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -477,7 +477,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
         let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
         let mut finished_map = FxHashMap::default();
 
-        for constraint in regions.constraints.keys() {
+        for (constraint, _) in &regions.constraints {
             match constraint {
                 &Constraint::VarSubVar(r1, r2) => {
                     {