Unify rustc and rustdoc parsing of cfg()

This extracts a new `parse_cfg` function that's used between both.

- Treat `#[doc(cfg(x), cfg(y))]` the same as `#[doc(cfg(x)]
  #[doc(cfg(y))]`. Previously it would be completely ignored.
- Treat `#[doc(inline, cfg(x))]` the same as `#[doc(inline)]
  #[doc(cfg(x))]`. Previously, the cfg would be ignored.
- Pass the cfg predicate through to rustc_expand to be validated

Co-authored-by: Vadim Petrochenkov <vadim.petrochenkov@gmail.com>

src/librustdoc/clean/mod.rs

@@ -2006,7 +2006,7 @@ fn clean_extern_crate(
         def_id: crate_def_id,
         visibility: krate.vis.clean(cx),
         kind: box ExternCrateItem { src: orig_name },
-        cfg: attrs.cfg(cx.sess().diagnostic()),
+        cfg: attrs.cfg(cx.sess()),
     }]
 }