Encode proper spans in crate metadata.

The spans previously encoded only span the first token after the opening
brace, up to the closing brace of inline `mod` declarations. Thus, when
examining exports from an external crate, the spans don't include the
header of inline `mod` declarations.

src/librustc_metadata/encoder.rs

@@ -563,7 +563,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
         Entry {
             kind: EntryKind::Mod(self.lazy(&data)),
             visibility: self.lazy(&ty::Visibility::from_hir(vis, id, tcx)),
-            span: self.lazy(&md.inner),
+            span: self.lazy(&tcx.def_span(def_id)),
             attributes: self.encode_attributes(attrs),
             children: self.lazy_seq(md.item_ids.iter().map(|item_id| {
                 tcx.hir.local_def_id(item_id.id).index