Generate docs for links to private items when passed --document-private

- Pass around document_private a lot more
- Add tests
  + Add tests for intra-doc links to private items
  + Add ignored tests for warnings in reference links

src/test/rustdoc-ui/intra-links-private.rs

@@ -0,0 +1,10 @@
+// check-pass
+// revisions: public private
+// [private]compile-flags: --document-private-items
+#![cfg_attr(private, deny(intra_doc_resolution_failure))]
+
+/// docs [DontDocMe]
+//[public]~^ WARNING `[DontDocMe]` public documentation for `DocMe` links to a private item
+// FIXME: for [private] we should also make sure the link was actually generated
+pub struct DocMe;
+struct DontDocMe;