Reuse existing shared Lrc for MatchImpl parent

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

Author: Mark-Simulacrum

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mismatched_static_lifetime.rs

@@ -33,7 +33,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
             ObligationCauseCode::MatchImpl(parent, impl_def_id) => (parent, impl_def_id),
             _ => return None,
         };
-        let binding_span = match **parent {
+        let binding_span = match parent.code {
             ObligationCauseCode::BindingObligation(_def_id, binding_span) => binding_span,
             _ => return None,
         };

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

@@ -189,7 +189,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         }
         if let SubregionOrigin::Subtype(box TypeTrace { cause, .. }) = &sub_origin {
             let code = match &cause.code {
-                ObligationCauseCode::MatchImpl(parent, ..) => &**parent,
+                ObligationCauseCode::MatchImpl(parent, ..) => &parent.code,
                 _ => &cause.code,
             };
             if let ObligationCauseCode::ItemObligation(item_def_id) = *code {

compiler/rustc_middle/src/traits/mod.rs

@@ -340,7 +340,7 @@ pub enum ObligationCauseCode<'tcx> {
     WellFormed(Option<WellFormedLoc>),
 
     /// From `match_impl`. The cause for us having to match an impl, and the DefId we are matching against.
-    MatchImpl(Lrc<ObligationCauseCode<'tcx>>, DefId),
+    MatchImpl(ObligationCause<'tcx>, DefId),
 }
 
 /// The 'location' at which we try to perform HIR-based wf checking.

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