Rollup of 4 pull requests Successful merges: - #62497 (Fix double resolving custom libdir) - #63209 (Stabilize `async_await` in Rust 1.39.0) - #63746 (Cherry-pick src/test changes with Centril's changes) - #63750 (rustc_metadata: replace LazySeq<T> with Lazy<[T]>.) Failed merges: r? @ghost