Get rid of item.span

- Remove `span` field, adding `Item::span()` instead
- Special-case `Impl` and `Module` items
- Use dummy spans for primitive items

src/librustdoc/passes/calculate_doc_coverage.rs

@@ -211,7 +211,7 @@ impl<'a, 'b> fold::DocFolder for CoverageCalculator<'a, 'b> {
                     None,
                 );
 
-                let filename = i.span().filename(self.ctx.sess());
+                let filename = i.span(self.ctx.tcx).filename(self.ctx.sess());
                 let has_doc_example = tests.found_tests != 0;
                 let hir_id = self.ctx.tcx.hir().local_def_id_to_hir_id(i.def_id.expect_local());
                 let (level, source) = self.ctx.tcx.lint_level_at_node(MISSING_DOCS, hir_id);