save-analysis: Get path def from parent in case there's no def for the path itself.

This fixes #57462.

The relevant part from the hir type collector is:

```
DEBUG 2019-01-09T15:42:58Z: rustc::hir::map::collector: hir_map: NodeId(32) => Entry { parent: NodeId(33), dep_node: 4294967040, node: Expr(expr(32: <Foo>::new)) }
DEBUG 2019-01-09T15:42:58Z: rustc::hir::map::collector: hir_map: NodeId(48) => Entry { parent: NodeId(32), dep_node: 4294967040, node: Ty(type(Foo)) }
DEBUG 2019-01-09T15:42:58Z: rustc::hir::map::collector: hir_map: NodeId(30) => Entry { parent: NodeId(48), dep_node: 4294967040, node: PathSegment(PathSegment { ident: Foo#0, id: Some(NodeId(30)), def: Some(Err), args: None, infer_types: true }) }
DEBUG 2019-01-09T15:42:58Z: rustc::hir::map::collector: hir_map: NodeId(31) => Entry { parent: NodeId(32), dep_node: 4294967040, node: PathSegment(PathSegment { ident: new#0, id: Some(NodeId(31)), def: Some(Err), args: None, infer_types: true }) }
```

We have the right ID when looking for NodeId(31) and try with NodeId(32) (which
is the right thing to look for) from get_path_data, but not for the segments
that we write from `write_sub_paths_truncated`.

Basically `process_path` takes an id which is always the parent, and that we
fall back to in `get_path_data()`, so we get the right result for the last path
segment, but not for the other segments that get written to from
`write_sub_paths_truncated`.

I think we can stop passing the explicit id around to `get_path_data` now, will
consider sending that as a followup.

Author: emilio

src/librustc_save_analysis/lib.rs

@@ -622,9 +622,11 @@ impl<'l, 'tcx: 'l> SaveContext<'l, 'tcx> {
             Node::Visibility(&Spanned {
                 node: hir::VisibilityKind::Restricted { ref path, .. }, .. }) => path.def,
 
-            Node::PathSegment(seg) => match seg.def {
-                Some(def) => def,
-                None => HirDef::Err,
+            Node::PathSegment(seg) => {
+                match seg.def {
+                    Some(def) if def != HirDef::Err => def,
+                    _ => self.get_path_def(self.tcx.hir().get_parent_node(id)),
+                }
             },
             Node::Expr(&hir::Expr {
                 node: hir::ExprKind::Struct(ref qpath, ..),