Remove path_span argument to the visit_path_segment methods.

The `visit_path_segment` method of both the AST and HIR visitors has a
`path_span` argument that isn't necessary. This commit removes it.

There are two very small and inconsequential functional changes.

- One call to `NodeCollector::insert` now is passed a path segment
  identifier span instead of a full path span. This span is only used in
a panic message printed in the case of an internal compiler bug.

- Likewise, one call to `LifetimeCollectVisitor::record_elided_anchor`
  now uses a path segment identifier span instead of a full path span.
  This span is used to make some `'_` lifetimes.

compiler/rustc_privacy/src/lib.rs

@@ -1315,7 +1315,7 @@ impl<'tcx> Visitor<'tcx> for TypePrivacyVisitor<'tcx> {
             }
         }
 
-        intravisit::walk_qpath(self, qpath, id, span);
+        intravisit::walk_qpath(self, qpath, id);
     }
 
     // Check types of patterns.