Add variant_name function to LangItem

Clippy has an internal lint that checks for the usage of hardcoded def
paths and suggests to replace them with a lang or diagnostic item, if
possible. This was implemented with a hack, by getting all the variants
of the `LangItem` enum and then index into it with the position of the
`LangItem` in the `items` list. This is no longer possible, because the
`items` list can't be accessed anymore.

compiler/rustc_hir/src/lang_items.rs

@@ -95,6 +95,14 @@ macro_rules! language_item_table {
                 }
             }
 
+            /// Returns the name of the `LangItem` enum variant.
+            // This method is used by Clippy for internal lints.
+            pub fn variant_name(self) -> &'static str {
+                match self {
+                    $( LangItem::$variant => stringify!($variant), )*
+                }
+            }
+
             pub fn target(self) -> Target {
                 match self {
                     $( LangItem::$variant => $target, )*