Implement associated lang items

Fixes #70718

This commit allows making associated items (e.g. associated functions
and types) into lang items via the `#[lang]` attribute. This allows such
items to be accessed directly, rather than by iterating over the parent
item's associated items.

I've added `FnOnce::Output` as a lang item, and updated one old usage to
use the new lang item. The remaining uses can be updated separately.

src/test/ui/assoc-lang-items.rs

@@ -0,0 +1,21 @@
+#![feature(lang_items)]
+
+trait Foo {
+    #[lang = "dummy_lang_item_1"] //~ ERROR definition
+    fn foo() {}
+
+    #[lang = "dummy_lang_item_2"] //~ ERROR definition
+    fn bar();
+
+    #[lang = "dummy_lang_item_3"] //~ ERROR definition
+    type MyType;
+}
+
+struct Bar;
+
+impl Bar {
+    #[lang = "dummy_lang_item_4"] //~ ERROR definition
+    fn test() {}
+}
+
+fn main() {}

src/test/ui/assoc-lang-items.stderr

@@ -0,0 +1,27 @@
+error[E0522]: definition of an unknown language item: `dummy_lang_item_1`
+  --> $DIR/assoc-lang-items.rs:4:5
+   |
+LL |     #[lang = "dummy_lang_item_1"]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ definition of unknown language item `dummy_lang_item_1`
+
+error[E0522]: definition of an unknown language item: `dummy_lang_item_2`
+  --> $DIR/assoc-lang-items.rs:7:5
+   |
+LL |     #[lang = "dummy_lang_item_2"]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ definition of unknown language item `dummy_lang_item_2`
+
+error[E0522]: definition of an unknown language item: `dummy_lang_item_3`
+  --> $DIR/assoc-lang-items.rs:10:5
+   |
+LL |     #[lang = "dummy_lang_item_3"]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ definition of unknown language item `dummy_lang_item_3`
+
+error[E0522]: definition of an unknown language item: `dummy_lang_item_4`
+  --> $DIR/assoc-lang-items.rs:17:5
+   |
+LL |     #[lang = "dummy_lang_item_4"]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ definition of unknown language item `dummy_lang_item_4`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0522`.