Auto merge of #81405 - bugadani:ast, r=cjgillot

Box the biggest ast::ItemKind variants

This PR is a different approach on https://github.com/rust-lang/rust/pull/81400, aiming to save memory in humongous ASTs.

The three affected item kind enums are:
 - `ast::ItemKind` (208 -> 112 bytes)
 - `ast::AssocItemKind` (176 -> 72 bytes)
 - `ast::ForeignItemKind` (176 -> 72 bytes)

compiler/rustc_builtin_macros/src/lib.rs

@@ -2,6 +2,8 @@
 //! injecting code into the crate before it is lowered to HIR.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(box_patterns)]
+#![feature(box_syntax)]
 #![feature(bool_to_option)]
 #![feature(crate_visibility_modifier)]
 #![feature(decl_macro)]