1499c089df
Introduce lowering_arena to avoid creating AST nodes on the fly `@oli-obk` requested this and other changes as a way of simplifying https://github.com/rust-lang/rust/pull/101345. This is just going to make the diff of https://github.com/rust-lang/rust/pull/101345 smaller. r? `@oli-obk` `@cjgillot`