Add annotations to the graphviz region graph on region origins

This adds
- (ex) for regions whose origin is existential,
- (p) for regoins whose origin is a placeholder, and
- (p for <name>) if the originating placeholder is named.

This has helped _my_ debugging and it doesn't create too bad clutter, I feel.

The change is ridiculously small, but I turned it into a separate PR so we can bikeshed the format.

compiler/rustc_borrowck/src/region_infer/graphviz.rs

@@ -41,7 +41,21 @@ fn render_region_vid<'tcx>(
         "".to_string()
     };
 
-    format!("{:?}{universe_str}{external_name_str}", rvid)
+    let extra_info = match regioncx.region_definition(rvid).origin {
+        NllRegionVariableOrigin::FreeRegion => "".to_string(),
+        NllRegionVariableOrigin::Placeholder(p) => match p.bound.kind {
+            ty::BoundRegionKind::Named(def_id) => {
+                format!("(p for {})", tcx.item_name(def_id))
+            }
+            ty::BoundRegionKind::ClosureEnv | ty::BoundRegionKind::Anon => "(p)".to_string(),
+            ty::BoundRegionKind::NamedAnon(_) => {
+                bug!("only used for pretty printing")
+            }
+        },
+        NllRegionVariableOrigin::Existential { .. } => "(ex)".to_string(),
+    };
+
+    format!("{:?}{universe_str}{external_name_str}{extra_info}", rvid)
 }
 
 impl<'tcx> RegionInferenceContext<'tcx> {