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 when we look 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 as a followup.
edec83835a