Mark allowed dead code and lang items as live

Dead code pass now explicitly checks for `#[allow(dead_code)]` and
`#[lang=".."]` attributes on items and marks them as live if they have
those attributes. The former is done so that if we want to suppress
warnings for a group of dead functions, we only have to annotate the
"root" of the call chain.

Close #11380 and #11440.