Parse lightweight clones syntax

[dtolnay]

• RFC: Simplify lightweight clones, including into closures and async blocks rust-lang/rfcs#3680
• Tracking issue: Tracking issue for ergonomic reference counting rust-lang/rust#132290
• Feature gate: #![feature(ergonomic_clones)]

f(obj.use);
thread::spawn(use || f(obj));
task::spawn(async use { f(obj).await });