rustdoc: clean QPath::LangItem

This commit adds support for cleaning `QPath::LangItem` and
`hir::GenericBound::LangItemTrait` in rustdoc. `QPath::LangItem`
does not require lowering, and `hir::GenericBound::LangItemTrait`
is lowered to a `GenericBound::TraitBound`.

Signed-off-by: David Wood <david@davidtw.co>

src/librustdoc/clean/utils.rs

@@ -335,6 +335,7 @@ pub fn qpath_to_string(p: &hir::QPath<'_>) -> String {
     let segments = match *p {
         hir::QPath::Resolved(_, ref path) => &path.segments,
         hir::QPath::TypeRelative(_, ref segment) => return segment.ident.to_string(),
+        hir::QPath::LangItem(lang_item, ..) => return lang_item.name().to_string(),
     };
 
     let mut s = String::new();