Reuse existing shared Lrc for MatchImpl parent

This is hopefully a small performance win for the hot path.

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2025,7 +2025,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let cause = ObligationCause::new(
             obligation.cause.span,
             obligation.cause.body_id,
-            ObligationCauseCode::MatchImpl(Lrc::new(obligation.cause.code.clone()), impl_def_id),
+            ObligationCauseCode::MatchImpl(obligation.cause.clone(), impl_def_id),
         );
 
         let InferOk { obligations, .. } = self