mask compiler_builtins docs

Since `compiler_builtins` is being injected automatically, its docs
aren't masked. This commit masks the crate's docs if it's brought in as
an extern crate.

src/librustdoc/clean/mod.rs

@@ -165,7 +165,12 @@ impl<'a, 'tcx, 'rcx> Clean<Crate> for visit_ast::RustdocVisitor<'a, 'tcx, 'rcx>
  match module.inner {
  ModuleItem(ref module) => {
  for it in &module.items {
- if it.is_extern_crate() && it.attrs.has_doc_flag("masked") {
+ // `compiler_builtins` should be masked too, but we can't apply
+ // `#[doc(masked)]` to the injected `extern crate` because it's unstable.
+ if it.is_extern_crate()
+ && (it.attrs.has_doc_flag("masked")
+ || self.cx.tcx.is_compiler_builtins(it.def_id.krate))
+ {
  masked_crates.insert(it.def_id.krate);
  }
  }