rustc_metadata: add a helper macro for recording into PerDefTable's.

Author: eddyb

src/librustc_metadata/decoder.rs

@@ -481,7 +481,7 @@ impl<'a, 'tcx> CrateMetadata {
     }
 
     fn maybe_entry(&self, item_id: DefIndex) -> Option<Lazy<Entry<'tcx>>> {
-        self.root.entries_table.lookup(self.blob.raw_bytes(), item_id)
+        self.root.per_def.entry.lookup(self.blob.raw_bytes(), item_id)
     }
 
     fn entry(&self, item_id: DefIndex) -> Entry<'tcx> {